Fetal Development Guideline: Stages of Development of the Fetus


Most people think child development starts after birth but the prenatal period has a lot to do with the overall developing process. As your body adjusts to accommodate pregnancy, your unborn child takes center stage – a reason that prompts us to share with you the different stages of fetal development.

In this guideline, we have combined everything pertaining the stages of fetal development so that you can know what happens in each week, month and trimester. As you read, you will come across the following, all arranged according to when they are supposed to happen:

  • Three stages of fetal development:Germinal, embryonic and fetal periods
  • Fetal development week by week:What happens in each pregnancy week?
  • Carnegie stages of fetal development:Based on the Carnegie system, 23 stages of embryonic development occur from week 1 to 8. They are based on the external features of the embryo. There will be a table to show the Carnegie stages after the 42 weeks overview.
  • Fetal development month by month:You will see how old the baby will be at the end of each week based on the months after the fourth week.
  • Fetal development chart:It summarizes what happens in each week.

First Trimester Of Pregnancy: Fetal Development

This is where it all begins. After baby making with your partner, you start getting used to becoming pregnant as symptoms begin to knock. In the 1st trimester, we will focus on the three main stages of fetal development with the last one encompassing the last part of the first trimester and the last two trimesters.

Let’s see what happens during the first 13 weeks.

The Germinal Stage of Pregnancy

It occurs in the first two weeks when the sperm meets the ovum in one (or both) of the fallopian tubes. After fertilization, the egg transforms to a zygote before a week’s journey to the uterus. After you conceive, cell division takes place in the next 24-36 hours.

Fetal Development Guideline

As it travels to the uterus, the process of growth and cell division takes precedence. The zygote splits to form double the amount of cells present which means it will first divide into two, then four, eight and so on – a process referred to as mitosis.

After the early cell division, most of the zygotes do not go past the initial cell division, and almost half of the zygotes formed do not survive past the second week.  Upon reaching the eight-cell point, cell differentiation starts taking toll where the cells take specific characteristics that determine what they become in future.

As multiplication of cells progresses, they also separate to form two masses with the outer cells forming the placenta, and the inner ones become the embryo. Cell division goes on at a rapid rate to form a blastocyst which has three layers:

  • Ectoderm:It eventually develops the nervous system and skin.
  • Endoderm:They form the respiratory and digestive systems.
  • Mesoderm:They eventually form the skeleton and muscles.

When the blastocyst reaches the uterus zone, implantation occurs where it attaches to one of the wall’s sides or on top. After implantation, you will notice spotting since the process ruptures tiny blood vessels in the uterus. On the other hand, the blood vessels web that forms around the blastocyst will be responsible for nourishing the cells from that point to delivery day.

One thing you need to note is that implantation is not always a sure process. According to research, nearly 60% of the natural conceptions that happen never proceed to a developing baby due to improper implantation in the uterus. All this time, the mother may not even be aware that she is pregnant.

Now that you are aware of what happens during the germinal stage let us see what week one and two holds for you and all the other urging-to-pregnant ladies out there.

1st-week Ultrasound

Well, unless you have been trying so hard to get pregnant, ultrasounds do not usually happen in week 1. For those who have been seeing a fertility specialist, you may have one to check the follicles in your ovaries, uterus lining thickness, and presence or absence of fibroids. If you have not conceived yet, the specialist will recommend the necessary measures to help you get pregnant.

For the healthy mothers-to-be, most of you will still be enjoying lovemaking during this period.

2nd-week Ultrasound

If you had an opportunity to see what is happening in the womb, your ovary would be releasing an egg to the fallopian tube which has to be fertilized in the next 24 hours. Since you are trying to get pregnant, then it means one of the sperms will do the honors and in case of two sperms fertilizing, get ready to have identical twins, but that will be revealed later.

Another possibility is fertilization of two eggs in both ovaries. In that case, you will have twins that are not identical.

The Embryonic Stage of Pregnancy

From the third to eighth week, the cells formed now become the embryo with distinctive qualities just like humans. It is a crucial stage that involves brain development. What happens is that the embryo will start dividing to form three layers which will all build up to essential body parts. The neural tube avails in the next 22 days after conception.

The tube will later progress to form the central nervous system which includes a brain and spinal cord. The neural tube development occurs along the neural plate area. Early signs show the presence of two ridges on each side. After a few days, more ridges develop and fold to form a hollow tube.

Once the tube is available, cells start to develop almost at the center as the tube closes to

Fetal Development Guideline

give way to the formation of brain vesicles. These vesicles are what will later constitute the major brain structures such as rear, mid and forebrain parts.

In the fourth week, the head starts to develop before the nose, ears, eyes, and mouth. The heartbeat begins as early as when the cardiovascular system starts to form where a blood vessel that eventually develops into a heart starts to pulse.

After the fifth week, buds that will develop into legs and arms become available. By the end of the eighth week, all the primary organs apart from those defining the sex are all available. If you are wondering, even the knees and elbows are there, it’s just that they are too tiny to notice. The overall weight of the embryo at this point is about a gram and one inch tall.

According to research, the neurons begin to form after 42 days from conception date and is almost complete during mid-pregnancy. As they generate, they move to different parts of the brain. After reaching the appropriate location, they start connecting with neural cells to form the underlying neural network.

After knowing what happens during the embryonic stage, let us see what an ultrasound would show from the third to the eighth week.

3rd-week Ultrasound

At this point, fertilization has already happened. The egg is now traveling through the fallopian tube as it multiplies to form two identical cells. An ultrasound may not detect all this due to the size of the egg – too small and at this point, it is referred to as a marula.

4th-week Ultrasound

During this period, it is most probably when you are expecting a missing menstrual period.

The doctor can see how the uterine wall is thickening which is a sign of implantation. TheFetal Development Guideline ball of cells is now dividing to form the embryo and placenta. Also, the brain and spine building blocks are also available. It is also in week four that the amniotic sac and fluid develop into a cushion responsible for protecting the baby. All this is there in the uterus as a tiny dot.

When the OB knows that you are four weeks pregnant, he or she may help you arrange for an appointment at the end of the month. If you have no health issues, then the only thing remaining is to start eating healthy and relax.

What is the baby’s size? Poppy seed is larger than the blastocyst in your uterus. It is busy settling in the new environment – the womb and the implantation point will define its position for the next nine months.

5th-week Ultrasound

In week 5, an ultrasound shows something that looks like a tadpole. Well, nothing may seem to happen, but major organs are developing – the stomach, kidneys, liver, and heart. Other systems that avail includes the nervous, digestive and circulatory.

Lack of medical history complications denies you a chance to see what the activities in the womb are constituting. Instead, the OB will push the ultrasound view to the eighth or ninth week. So, be patient!

What is the baby’s size? The size can be compared to what an apple seed measures. It is about 0.13 inches in length from the head to rear.

Month’s calculation: At five weeks, the baby is now about a month old. If you thought you are way past two months that is because the specialists start counting after the last period. Now you have eight more months of preparing and waiting.

6th-week Ultrasound

In the sixth week, a visual in the uterus will show how the circulatory system has developed into a more sophisticated one. The tiny hands and feet may also be wiggling during this period. Since it somehow bigger than the last week, a nose, ears, eyes, cheeks, and chin will start to sprout. The doctor might also be able to detect a fetal heartbeat which will indicate something alive is growing further. If not detectable, don’t worry, there is still time and room for development.

If you have twins, then a visual of two gestational sacs will be present. They form between the 4th and 10th day after conceiving.

What is the baby’s size? The measurement stands at about 0.25 inches, the same size as a sweet pea which is bound to double in the next week.

Month’s calculation: The baby is now about a month and a week old.

7th-week Ultrasound

At this time, the brain is rapidly developing with a hundred or more cells forming every other minute. The heart is also gaining as permanent kidneys continue to build. The leg and arm joints will also be taking shape.

At this time, you will be preparing for the first prenatal visit which will confirm if your baby is okay after the ultrasound, blood and urine tests that may occur when attending to the appointment. The specialist may also be able to calculate your due date based on the size and nature of the developing embryo.

For the twins case, early signs may show you but still hard to predict without actually seeing it. If you have higher levels of hCG than usual during a check, that could be a leading signal that you have more than one baby developing. Also, an earlier than usual or severe morning sickness could lead to twins results.

What is the baby’s size? At about 0.51 inches, the baby can be compared to a blueberry – double from last week.

Month’s calculation: The baby is now a month and almost two weeks old.

8th-week Ultrasound

Now, it is time for your prenatal checkup which means you can finally get a glimpse of what is happening.  At this point, the arms and legs are moving, but you cannot feel it yet. It is also during this week that you see the webbed toes and fingers and the extended part on the rear that looks like a tail is now gone. The funny part is that the taste buds are currently developing this early.

During the first prenatal visit, the doctor will draw blood from you for some laboratory

Fetal Development Guideline

tests. The first thing to check is if you are Rh negative or positive. If the result is different from what your baby has, the doctor will recommend the appropriate medical precautions to prevent future complications. The physician will also make sure that blood cells and hormone levels are okay. There is also a check against STDs, Hepatitis B, and HIV/AIDS among other immunities.

One more thing, you will get a pap smear to check for infections not to mention the peeing protocol which will be the new norm in almost every other appointment so that the doctor can check the protein and glucose levels.

What is the baby’s size? The baby can be compared to raspberry with weight and height measurements reading about 0.04 ounces and 0.63 inches respectively. The growth rate is about a millimeter per day.

Month’s calculation: Almost two months pregnant.

The Tetal Stage of Pregnancy

After the completion of cell division, the embryo graduates to a fetus. It is during this period that essential changes in the brain take place. We will focus on this period from now to the end since it starts from the ninth week to birth.

The organs and structures that commenced during the embryonic stage continue to develop in this phase. Also, we will see how the neural tube progresses to form the brain and spinal cord. The already built neurons migrate to their expected location before synapses begin – the connection of neurons.

Between week 9 and 12, the earliest form of reflexive motions starts to occur when it comes to the legs and arms.

This is the longest stage of development as you might have guessed and it is marked by tremendous changes as the fetus grows in preparation for birth. In the third month, sex organs start to take their distinct form, and by the time three months are over, all body parts are already there. The overall weight at this point is about 3 ounces as the fetus continue to grow. However, more physical changes will occur during later stages.

The first trimester ends after the third month, and during the next three months of the second trimester, the heartbeat continues to go viral as other systems continue to take form. It is also during the second trimester that the hair, finger, and toenails among other parts develop and take shape. By the end of the sixth month, the fetus will be six times larger than the size at the end of three months.

We earlier highlighted brain development during this stage. It is in the second trimester that you will see a conscious brain and by the 28th week, the brain’s activity can be compared to that of a sleeping young one after birth.

Form the seventh month until birth; the fetus develops by adding more weight and preparing for the outside environment after birth. We will also see the lungs developing as the fetus practices to breathe with the help of the already generated muscles.

9th-week Ultrasound

Most of the first time prenatal visits happen between the 8th-12th weeks. If an ultrasound occurred during the ninth week, one of the exciting things to see is the tiny heartbeat. During this week, the ultrasound is transvaginal implying that a probe will be inserted via your vagina since the uterus is still behind the pelvic bone.

You will see a tiny fetus that looks like lima bean, but you will get a confirmation that the pregnancy is healthy and not an ectopic. At week 9, the physician will say things like the yolk sac, gestational sac, and a fetal pole. Since the embryo has now transformed into a fetus, the organs that started developing during the embryonic stage are now beginning to gain in size and complexity.

Back in the belly, the baby’s facial features are developing into a more distinctive form and heartbeat is getting stronger to a detectable point. It is around this time that many women think about a miscarriage. However, if the pulse is there, the risk of losing your pregnancy falls to 3%. It goes on low as the pregnancy progresses.

What is the baby’s size? At nine weeks, the baby is as big as a cherry. The height and weight measurements include 0.9 inches and 0.07 ounces respectively.

Month’s calculation: The baby is now two months old.

10th-week Ultrasound

Fetal Development Guideline

During the tenth week, the fetus has working joints in the arm as the bones and cartilage continue to develop. The vital organs are now good to go and already starting to function. It is during this week that the nails and hair begin to grow as the fetus performs some swallowing practices. The kicks are more robust than the previous week too.

If you ask a gender counselor about the genetic makeup, the testing is optional at this juncture, but he or she can help you in deciding after a look at the risk factors involved plus your family history.

What is the baby’s size? At week 10, your baby can be compared to a strawberry. The fetus now measures about 0.14 ounces for the weight with an average height of 1.2 inches.

Month’s calculation: the baby is two months and one week old.

11th-week Ultrasound

As the baby moves gracefully inside the belly, the skin is translucent during this week and working toward being opaque. The toes and fingers are unwebbed, and the fetus is also developing hair follicles, tooth buds, and nailbeds.

Between now and the 13th week, you will be experiencing the first screening. It is usually a combination of a blood test and NTS (Nuchal Translucency Screening). An ultrasound happening during this week will involve measuring the fetus’ back neck where an unusual measurement could be a leading signal to a chromosomal disorder. The blood test is there to reveal the hormone levels. Results from both procedures will direct the doctor on what to tell you regarding the chromosomal complication, but you have to wait.

If you have twins, the 11th-week ultrasound will show two umbilical cords and either one or two placentas. If they share a placenta, you are most likely to have identical twins. For the separate placenta case, the results could be fraternal or identical. Having twins means frequent checkups to make sure that they are both healthy and getting adequate nutrients.

What is baby’s size? The baby can now be compared to a lime’s size. The fetus weighs about 0.25 ounces with a height of about 1.6 inches. The head is still bigger with a ratio of 1:1 when compared to the body.

Month’s calculation: The baby is now two months and two weeks old. The following ultrasound shows baby situation.

12th-week Ultrasound

If your insurance covers for an ultrasound every other week that you may wish to see your baby, the week 12 appointment will reveal a brighter image than what you saw in the previous weeks. During this period, the vital organs are almost done with the kickstart, and the only thing remaining is for them to develop to maturity. With the brain still in a rapid development schedule, it can now send signals to make the fetus open and close the fingers not to mention the curling toes.

The baby knows about reflexive action. If you can poke your stomach during an ultrasound, you might see the baby’s reaction. If you have twins, both fetuses will be developing at the same rate with the singletons, but the growth rate gradually goes down in the coming weeks. If anyone does not know yet, you can now spread the news that a new family member is on the way.

What is the baby’s size? He or she is comparable to a plum. The average weight and height are 0.49 ounces and 2.1 inches respectively.

Month’s calculation: The baby is now almost three months old.

13th-week Ultrasound

With the organs in the growth mode, an ultrasound during this week could tell the baby’s gender. If the doctor says that it is still hard to predict, that means that the sex organs are hard to predict at this point and you will have to wait until the 18th week onwards. If you happen to get a cell-free DNA to check for chromosomal disorders, the results can also define the gender.

What is the baby’s size? The fetus can be compared to lemon. Weight and height measurements are about 0.81 ounces and 2.9 inches respectively.

Month’s calculation: You are three months pregnant since the first trimester ends after a 3-month period.

Third Trimester Of Pregnancy: Fetal Development

During the second trimester, the baby is still tiny with more room for development. For most mothers, morning sickness is fading away as your body refreshes to the adjustments. In case you are wondering, this is the best time to think about exercising.

14th-week Ultrasound

An ultrasound is unlikely to happen during this week until the mid-pregnancy scan between

Stages of Development of the Fetus

the 18th and 22nd week. However, an inside visual during this time would show the wiggling fingers and toes, and the baby might even be sucking a thumb now.

The kidneys are now functioning by releasing urine, and the spleen and liver are also up to their tasks. At week 14, the earliest hair form (lanugo) is now growing to cover the body. At this point, it is still hard to determine the gender.

What is the baby’s size? The baby can be compared to a peach. Weight and height measurements include 1.5 ounces and 3.4 inches respectively. The fetus weight is almost double from last week.

Month’s calculation: The baby is now three months and one week old.

15th-week Ultrasound

You may not feel it yet, but the baby is wriggling harder now that the limbs are working. It is also at this week that the baby might be having hiccups. During this period, the doctor may advise you to take an MMS (Multiple Marker Screen) where blood is drawn to measure specific protein and hormone levels for accurate valuation of any neural tube complications.

An Amniocentesis is another test that can happen between now and the 2oth week. It is an invasive one which the doctor uses to diagnose, chromosomal disorders, neural tube complications and other genetic issues. You may opt for this procedure if the baby has potential genetic abnormalities that need checking and addressing.

Commonly referred to as an amnio, the doctor carefully inserts a needle into the amniotic sac to collect a sample fluid for testing.

What is the baby’s size? The baby is now comparable to a navel orange. Weight and height measurements are 2.5 ounces and 4 inches respectively. The head to body proportion is getting more even as the legs become longer the arms.

Month’s calculation: The baby is now three months and about two weeks old.

16th-week Ultrasound

The baby’s ears are developing, and due to the cartilage forming in this body parts, the fetus can hear your voice. More hair is growing to add to the eyelashes and brows. Taste buds are also on their way in.

If you have a prenatal appointment after four months during this time, there will be a urine test to check for preeclampsia and gestational diabetes. If the OB confirms a healthy child in a safe pregnancy after the ultrasound, the chances of getting a miscarriage fall to 1%.

What is the baby’s size? Compare the baby with avocado, and you get the most appropriate size. Height and weight measurements are about 4.6 inches and 3.5 ounces.

Month’s calculation: The baby is now three months and three weeks old.

17th-week Ultrasound

The baby’s cartilage is now transforming to the bone. More fat and meat is forming around the bones which implies that the fetus is getting stronger. The umbilical cord is getting thicker too.

An ultrasound can be conducted during this week if you choose to have a cordocentesis. It involves testing the umbilical cord for chromosomal disorders. With the help of an ultrasound, the doctor finds the part where the placenta meets the cord, and that is where he or she will draw blood from.

It is necessary to perform this procedure if the amniocentesis results were not entirely clear and you are looking for a more precise answer.

What is the baby’s size? The baby is as large as a pomegranate. The average height and weight measurements include 5.1 inches and 5.9 ounces respectively.

Month’s calculation: The baby is now three months and three weeks old.

18th-week Ultrasound

There is too much activity during this week. Your fetus is now moving a lot thanks to the developing muscles. The baby can now swallow, suck and yawn and you can feel the twists, turns and kicks performance.

If you are having twins, you will see the membrane that separates the two fetuses. As they

wriggle, they push the fluid on the layer making it to slightly shift, something that you might witness during the ultrasound.

In this mid-pregnancy checkup, there is a detailed baby examination to make sure everything is growing as intended. The baby is measured, and the major organs are also checked. Other things that the doctor looks at include the placenta’s location, amniotic fluid, and the heart rate.

If you have twins or triplets, the anatomy will involve all the babies. Do you still want to know the gender? There is a 95% chance that the doctor can tell whether it is a boy or a girl if the baby’s position allows the specialist to see. If the baby does not cooperate, there is still time.

What is the baby’s size? The fetus can be compared to an artichoke. Weight and height measurements are about 6.7 ounces and 5.6 inches respectively. If you are feeling hungrier during this period, well, the baby is growing bigger and quicker.

Month’s calculation: The baby is now four months old.

19th-week Ultrasound

The fetus is now growing a protective layer on the skin which is known as vernix caseosa. It is white in color and also greasy, and the baby might have some of it during delivery. It is also during this period that the baby is working on the five common senses. That means that the brain is developing the nerve cells for hearing, sight, smell, and taste.

During this week, it is also recommendable that you start searching for a pediatrician. Ask around before getting the one that you will ask all the questions from this moment to when the baby reaches their first year after birth.

What is the baby’s size? At 19 weeks, the baby can be compared to a mango. Height and weight measurements include about 6 inches and 8.5 ounces respectively.

Month’s calculation: The baby is now four months and one week old. Please watch the 19th week of your pregnancy.

20th-week Ultrasound

The fetus can now taste thanks to the already developed taste buds and is swallowing the amniotic fluid every other day. A mid-pregnancy ultrasound should be happening now if not yet since the baby is transforming rapidly. The heart is now chambered, the kidneys are functioning, and the brain is now in hemispheres. The physician checks everything to makes sure there are no risk factors.

For the twins’ case, the babies are checked to compare the sizes. If the measurements differ, then more is done to make sure that neither is having problems as they grow. If you still want to see if the Chinese gender chart concurs with what you are carrying, the answer is more vivid during this week onwards.

What is the baby’s size? The baby is now the same size as a banana. Weight and height measurements include about 10.2 ounces and 6.5 inches respectively. From the next week, the measures will be from head to toe.

Month’s calculation: The baby is now four months and two weeks old.

21st-week Ultrasound

Well, a lot is happening now. The digestive system is now ready, and the fetus is now making a black substance known as meconium. It the first thing the baby will excrete after birth. Well, have you thought about your future generation? If you are carrying a girl, there are about 6 million eggs in the ova. If you have a boy, the testes are still in the abdomen and will drop soon as the pregnancy progresses.

Since this is probably the last ultrasound, make sure you have some photos printed after the session if you want to keep the memory.

What is the baby’s size? The baby can now be compared to an endive’s head. Weight and height measurements include 12.7 ounces and 10.5 inches respectively. The baby is now large enough to make you feel the kicks every time they launch.

Month’s calculation: The baby is now four months and three weeks old.

22nd-week Ultrasound

During this week, the baby resembles a newborn. There are lips and eyes present, and the fetus now has sleeping cycles – 12-14 hours in a day. So, if at certain times you do not hear any kicks, maybe someone is asleep.

Stages of Development of the Fetus

This is probably the last week if you are expecting a mid-pregnancy ultrasound check so you will get to see all the major and developed organs. By now, the gender should be clear to you.

What is the baby’s size? The baby is as big as a coconut at week 22. Height and weight measurements include about 15.2 ounces and 10.9 inches respectively. Remember that these measurements are from head to toe from now on.

Month’s calculation: We can say your baby is now four months and four weeks old, but after this week, it will be officially five months. 4 more to go!

23rd-week Ultrasound

Inside the belly during this week shows more progress. Nipples are forming during this period, and the face is complete only to need some thickening fat. The baby can hear your voice now, plus any other loud source such as a barking dog or frenzy arguments.

You will be seeing the doctor once in a month from now until you reach the third trimester. Since you have more energy now than during later stages, make sure every other preparation that you need to attend to is done.

What is the baby’s size? The baby can be compared to a grapefruit. Weight and height measurements are about 1.1 pounds and 11.4 inches respectively. At almost a foot long, the weight is now measurable in pounds.

Month’s calculation: You are now five months pregnant.

24th-week Ultrasound

There is more baby progress here. The baby’s skin is getting more opaque, and with capillaries forming beneath the surface, the fetus now has a pink glow.

During this week, the doctor will engage you in a glucose challenge screening test to check if you have gestational diabetes. You will be asked to drink Glucola, a sweet liquid that is used to see how your body works on the sugar intake. After an hour of sitting, the physician will draw blood and test for the sugar processing.

If there are complications, the next test will be to check for glucose tolerance. How your body processes the sugar will be monitored over a three-hour period to see if you have a gestational diabetes risk. If the results are positive for the condition, the doctor will give you the appropriate counsel in a bid to regulate the problem.

The baby will also experience regular monitoring, so you will have a chance to see the baby once more.

What is the baby’s size? The baby is as big as a cantaloupe. Weight and height measurements include about 1.3 pounds and 11.8 inches respectively.

Month’s calculation: The baby is now five months and one week old.

25th-week Ultrasound

During this week, the baby is getting to understand the equilibrium status. He or she is learning the up and down positions as more fat and hair continue to accumulate.

You do not have an ultrasound at week 25 unless there is a reason for more screening. Until the 28th week when the checkup will increase to once or twice per week, you will be seeing the doctor once in a month.

If you did not have the glucose challenge last week, this is another window for the doctor to perform the procedure.

What is the baby’s size? The baby can be compared to the head of cauliflower. Weight and height measurements include about 1.5 pounds and 13.6 inches respectively.

Month’s calculation: The baby is now five months and two weeks old.

26th-week Ultrasound

You don’t get an ultrasound during this week, but if you had a chance to see what is happening, the baby is now inhaling amniotic fluid since there is no air – a practice meant for outside world preparation.

The immune system is gearing up too, and the antibodies are flowing from your body to the fetus. Eyes are still forming, and they will soon open up now that the eyelashes are fully grown.

What is the baby’s size? The baby can be compared to the kale’s head. Height and weight measurements include about 14 inches and 1.7 pounds respectively.

Month’s calculation: The baby is now five months and three weeks old.

27th-week Pregnancy

At week 27, the baby is busy practicing how to breathe as the lungs continue to develop rapidly. There is more brain activity during this period as it gains complexity.

 fetus Stages of Development

If there are no issues with your pregnancy, then you will not have an ultrasound at week 27. Since you will be preparing for more doctor appointments after the 28th week, it is time to grab any material that will give ideas on preparing for parenthood.

What is the baby’s size? The baby can be compared to a lettuce’s head. Height and weight measurements include about 12.4 inches and 1.9 pounds respectively.

Calculation by months: The baby is now five months and four weeks old. Now that you are in the sixth month, this is second trimester’s last week.

Third Trimester Of Pregnancy: Fetal Development

Now that you are entering the last phase of pregnancy, you will be feeling less energetic from now on, and you will note the frequent visits to the bathroom. It is time to prepare for a birth class now that the pregnancy honeymoon is over.

28th-week Ultrasound

At week 28, the baby is currently developing more fat underneath the skin, so it is now getting smoother. The baby is still practicing how to breathe now that the lungs are fully developed. If the baby was born during this week, there are chances that he or she may survive. The baby is now getting bigger and smarter thanks to the developing brain. After a few finishing touches, the baby will be ready to join the world.

You will now be seeing the OB twice in a month. If you have no pregnancy complications, then there is no need for an ultrasound during this week.

What is the baby’s size? The baby is as big an eggplant. Height and weight measurements include about 14.8 inches and 2.2 pounds respectively.

Month’s calculation: The baby is now six months old.

29th-week Ultrasound

If an ultrasound happened during this week, the baby would be large enough for the womb such that you will hear every kick he or she performs. The fetus is also having hiccups in case you feel frequent twitching. Under the skin, white fat deposits are growing, and this energizes the growing unborn.

If you have twins, this week will show a membrane formed between the fetuses. There is more crowding in this case.

What is the baby’s size? The baby is as big as acorn squash. Weight and height measurements include 2.5 pounds and 15.2 inches respectively. The weight will be triple during delivery.

Month’s calculation: The baby is six months and one week old.

30th-week Ultrasound

There is no ultrasound conducted during this week, but a glimpse in the womb will show

Stages of Development of the Fetus

the skin getting smoother than before. The brain is wrinkling at week 30 to make room for the crucial brain tissue. Now, there is more energy to enable the fetus to grasp a finger.

Do you have twins? They will be growing at the same rate as singletons until week 32 when the growth rate tends to slow down.

If you are carrying twins, the physician may opt for a biophysical profile taken from the twosome. It involves ultrasound and an NST (non-stress test). For the latter, the doctor will put sensors on the belly to measure the babies’ contractions and heart rates. If everything is fine after the procedure, the doctor will approve your pregnancy as free from fetal distress meaning that the growth progress is okay and there is nothing to worry about.

What is the baby’s size? The baby is as big as a zucchini. Height and weight measurements include 15.7 inches and 2.9 pounds respectively. There will be about half a pound addition to the weight and height in the coming weeks.

Month’s calculation: The baby is now six months and two weeks old.

31st-week Ultrasound

More and more brain and nerve progress are happening during this week. The eyes are still growing, and the irises can detect light if you shine some on your belly. All the five senses are working as intended. Also, at week 31, the head is now facing down which you can confirm during your next appointment.

Unless you have twins or gestational diabetes among other pregnancy-related complications, you do not need any screening during this period.

If you can get a 3D/4D ultrasound, which is usually conducted between the 24th-32nd week, you will see the three-dimensional image of your baby sucking a thumb, in smiles or just frowned and blinking. You can collect a video to take home after the visual. It is, however, an expensive undertaking that the insurance may not cover for which means digging deeper into your pockets.

What is the baby’s size? Your baby can be compared to an asparagus’ bunch. Weight and height measurements include 3.3 pounds and 16.2 inches respectively. If you feel as if your lungs are crowded, that is because the baby is getting bigger which will make you feel tired even after a small talk as you walk.

Month’s calculation: The baby is now six months and three weeks old.

32nd-week Ultrasound

The baby is getting ready to descend since the head is now in a downward position. Due to growth in size, the fetus is more cramped than before.

You will probably have a checkup during this week since the OB is likely to see you often. You may not have an ultrasound unless you have issues that need addressing or if you are carrying twins.

What is the baby’s size? The baby can be compared to a squash. Weight and height measurements include about 3.8 pounds and 16.7 inches respectively.

Month’s calculation: The baby is now seven months old.

33rd-week Ultrasound

Inside your 33 week pregnancy, the results show that the baby can open his or her eyes

Stages of Development of the Fetus

when awake. There is more breathing, swallowing and sucking coordination than before. The bones are still on their way to harden, and the brain is developing to bring you a smarter kid.

An ultrasound during this week is unlikely to happen unless you have a highly-risked pregnancy condition which may require a biophysical profile (BPP) procedure (please refer to 30th-week ultrasound).

What is the baby’s size? The baby can be compared to a celery’s head. Weight and height measurements include about 4.2 pounds and 17.2 inches respectively.

Month’s calculation: The baby is now about seven months and one week old.

34th-week Ultrasound

At week 34, the central nervous system continues to mature with final touches being done on the lungs. During this week, the doctor may perform the BPP if you did not have one last week to make sure that the fetus is thriving and there are no stress hindrances.

What is the baby’s size? The baby is as big as butternut squash. Weight and height measurements include about 4.7 pounds and 17.7 inches respectively.

Month’s calculation: The baby is now about seven months and two weeks old.

35th-week Ultrasound

The baby can now hear high-pitched sounds thanks to the fully developed ears and hearing mechanism. If you are expecting a boy, an ultrasound during this period will show descended testes in the scrotum.

Between this week and the next, the doctor may conduct a Group B Strep Test where he or she will take samples from your rectum and vaginal areas to test against the Group B Strep bacteria. In case you are wondering, it does not render you unhealthy but will be harmful to the baby during delivery. If you have it, the doctor will issue the necessary medication to prevent the baby from exposure at birth.

What is the baby’s size? Your baby can be compared to pineapple at week 35. Height and weight measurements include about 18.2 inches and 5.3 pounds respectively. Before birth, the fetus will have gained an average of one or more pounds of fat.

Month’s calculation: The baby is now about seven months and three weeks old. The video is about a 35 week pregnancy baby.

36th-week Ultrasound

At week 36, the kidneys and liver are fully functional. The immune and circulation systems are also ready for life after the womb. With more breathing practices in the previous weeks, the baby is gaining more experience on how to breathe on its own. The skin is also getting smoother and softer than before not forgetting that the gums are becoming more firm.

In this week’s appointment, the prenatal checkup may call for seeing the baby’s position. If the fetus’ head is not facing down, then that is a breech baby. Still, there are chances that the baby will eventually turn. However, the physician may want to perform a version procedure for the breech case. It is not usually comfortable since it involves lifting or pushing or both of your already swollen belly. On the other hand, it is a low-risk procedure that works on a significant number of patients, and the doctor will issue the appropriate medication to help you and the uterus relax.

An ultrasound comes in handy during the procedure as the doctor will be using it to help in adjusting the baby. The baby’s heart rate is also measured before and after the rough task.

One more thing, if you did not have the Group B test last week, this is another week to have it done. Since about 10-30% of women test positive for the bacteria, the solution involves dripping an antibiotic during labor which helps reduce the risk of transmitting bacteria to your baby when delivering.

High-risk conditions that may compromise your pregnancy may also call for a BPP during this week’s prenatal appointment. An okay from the results will prompt the doctor to rule out the early delivery recommendation.

What is the baby’s size? Your baby can be compared to papaya. Weight and height measurements include about 5.8 pounds and 18.7 inches respectively.

Month’s calculation: The baby is now eight months old. One month to go!

37th-week Ultrasound

More practice on how to breathe, suck, blink and grip is this week’s norm. It is also during this period that the baby is getting ready for his or her first meconium discharge from the digestive system which implies proof of good for the diaper.

An ultrasound at week 37 could be available as part of a BPP. If you have twins, the doctor will suggest a Caesarian delivery or induction, but that will depend on the nature of the pregnancy and medical history. This could lead to giving birth after 38 weeks which means meeting the duo earlier than usual.

Regardless of what the doctor recommends, be easy if there assurance of safety and newborn survival.

What is the baby’s size? Your baby is as big as a romaine lettuce’s head. Weight and height measurements include 6.3 pounds and 19.1 inches respectively. Weight gain is now about ½ an ounce every day.

Month’s calculation: The baby is approximately eight months and one week old now.

38th-week Ultrasound

During this week’s ultrasound, the baby’s hair shows more growth with an average height of one inch. Do you remember the vernix caseosa back in the 19th week? It is now gradually shedding, but there will still be some of it during delivery.

Since you will have a weekly appointment with your OB, the specialist will check if the baby

is an upside down position and if the head is in the pelvic chamber region. You will also undergo a pelvic examination to check for opening and thinning aspects which are one of the leading signs of labor readiness.

However, the expansion and thinning factors have some limitation towards predicting being ready for labor since the pain could go for weeks or hours if not days. Regardless of whether you have dilated or not, labor could occur the next day, making it an unpredictable scenario.

If there were a thorough checkup on the baby, the doctor would conduct a BPP to check the heart rate, breathing, muscle tone, movements and the amniotic fluid. The results of the procedure will determine if you need to deliver earlier than expected.

What is the baby’s size? Your baby can be compared to a winter melon. Height and weight measurements include about 19.6 inches and 6.8 pounds respectively. The weight is closer to what is considered at birth, and the head’s circumference is almost the same as his or her abdomen.

Month’s calculation: The baby is now approximately eight months and two weeks old.

39th-week Ultrasound

This week’s ultrasound shows a baby can flex the limbs as the brain keep on developing. The nails may be slightly grown past the fingertips.

You may experience a non-stress test plus an ultrasound during this week to check for the overall wellbeing of the soon to be born child. It is especially necessary if you are carrying twins so that the doctor can rule on whether to keep waiting until labor or have an early delivery.

What is the baby’s size? Your baby can be compared to a pumpkin. Weight and height measurements include about 7.3 pounds and 20 inches respectively.

Month’s calculation: The baby is now about eight months and three weeks old. One week to 9 months of gestation!

40th-week Ultrasound

The nails and hair are still growing not to mention the lungs too.

At 40 weeks, the doctor is more likely to perform a BPP. You will have the non-stress test

procedure for your contractions, movements in the womb and the baby’s heart rate

Stages of Development of the Fetus

reaction. The ultrasound will be used to check the level of amniotic fluid.

After the whole procedure, an induction may follow, or you will have to wait if there are no complications. Furthermore, you are now close to giving birth.

What is the baby’s size? Your baby is as big as a watermelon. Weight and height measurements are approximately 7.6 pounds and 20.2 inches respectively.

Month’s calculation: The baby is now nine months old meaning it is time to prepare for labor and delivery.

41st-week Ultrasound

As you wait for the D-day, the baby has now turned full term already and is gaining more weight. The nails and hair are longer than last week – a reason as to why some kids are born with a head full of hair.

Still, a BPP can occur during this week to make sure you (mother and child) are both okay. Those around you will help you decide on whether to wait or induce.

What is the baby’s size? Your baby can still be compared to the watermelon. Height and weight measurements include approximately 20.4 inches and 7.9 pounds respectively.

Month’s calculation: The baby is nine months and one week old inside your belly. At times, it may proceed to the 42nd week, but after that, an induction becomes inevitable.

Labor and Birth

Finally, you will be meeting your newborn after nine whole months of waiting but what leads to this juncture is always a mystery, and this implies giving it time. You can verify on how to know you are in labor and educate yourself on the available delivery options among other vital aspects.

Carnegie Stages of Fetal Development

These stages are utilized by embryologists to help in describing the relative maturity of the embryos. There are 23 stages in total, and they are all based on the embryo’s external features.

This system is independent of the chronological age and the embryo’s size. The numbers are arbitrary ranks that are drawn from the present physical features. So, even though the embryos have different sizes and ages, they may be placed in the same stage due to the similarity in appearance given that individuals also have natural variation.

Based on what happens during each week, the Carnegie stages are as follows:

Carnegie Stage Number of days Size (mm)
1 1 (1st week) 0.1-0.15
2 2-3 0.1-0.2
3 4-5 0.1-0.2
4 5-6 0.1-0.2
5 7-12 (2nd week) 0.1-0.2
6 13-15 0.2
7 15-17 (3rd week) 0.4
8 17-19 1.0-1.5
9 19-21 1.5-2.5
10 22-23 (4th week) 2.0-3.5
11 23-26 2.5-4.5
12 26-30 3-5
13 28-32 (5th week) 4-6
14 31-35 5-7
15 35-38 7-9
16 37-42 (6th week) 8-11
17 42-44 11-14
18 44-48 (7th week) 13-17
19 48-51 16-18
20 51-53 (8th week) 18-22
21 53-54 22-24
22 54-56 23-28
23 56-60 27-31 

Fetal Development Chart

With all the above information, here is something that summarizes it all.

fetal development stage

  • Purple color represents development periods when major body structure flaws can occur.
  • Pink represents the development periods when minor structural flaws and major functional ones may occur.


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