Pregnancy Timeline: Fetal Development Week-by-Week for 40 Weeks


After trying to get pregnant, you may be wondering what is happening in you in a bid to counter the confusion and prepare in advance for the coming baby. The fetus develops after a viable egg is fertilized and cell division begins immediately after fertilization leading to growth. In most cases, categorization of pregnancy milestones involves the first, second and third trimesters.

Here is an overview of the fetal development timeline which reflects on what happens every other week after fertilization.  

Fetal Development: the First Trimester

3 Weeks

The two sets of cell nuclei containing DNA from you and your partner fuse together inside the egg after the sperm penetrates the egg. The fusion assigns your baby, now called a zygote, a gender, eye and hair color and more than 200 other genetically determined characteristics.

4 Weeks

Fetal Development Week-by-Week

The zygote implants itself on the uterus wall. As the fertilized egg grows into an embryo, a water-tight sac forms around it gradually filling with fluid. This is called the amniotic sac, and it acts as a cushion for the baby. Your baby also develops a placenta. The round, flat organ transfers nutrients from the mother to the baby and transports wastes from the baby to the mother’s system for discharge.

The first face takes form with large dark circles for the eyes. Facial features such as the mouth, lower jaw and throat start to develop.

5 Weeks

Your baby’s gastrointestinal tract and sensory organs begin to emerge. Bones begin to replace cartilage marking the beginning of the fetal bone development timeline. The neural tube (brain, spinal cord and other neural tissues of the central nervous system) is well-formed. The heart, blood and blood vessels also begin to develop. Your baby’s heart beats 65 times a minute which answers the question of when does a fetus develop a heartbeat. The embryo measures 1/16 – 1/8 inch long from the crown to the rump. Your baby now has a C-shaped bundle of cells.

6 Weeks

fetal development

Your baby’s facial features continue to develop now the eyes are forming. Each side of the head has little skin folds that will eventually become the ears. The baby is also developing tiny buds that will eventually grow into arms and legs. Little fingers and toes are building up too. In the body, something is pumping blood, and the baby’s heartbeat is detectable at this point. The neural tube that will constitute the future spine is closing up. Your baby has a translucent skin and is about 1/4 inch long.

7 Weeks

The head is usually 1/3 of the whole embryo which is now 1/3 of an inch. Little fingers start to form like a paddle, and soon, the hands will be out. The brain and facial features such as nostrils and eye lenses are growing at a high rate.

8 Weeks

At eight weeks, neurons are connecting to form brain network. Little fingers and toes begin to develop and are webbed as the arms and legs continue to grow. Organs such as the lungs, upper lip, eyes, nose, and ears start to avail. The fetus body is also straightening up, and small movements are happening, but you cannot detect them yet. The embryo now develops into a fetus and is about half an inch long.

9 Weeks

The baby’s heart is nearly complete. Eyelids and beginnings of teeth are forming just like the nipples and hair follicles. Arms are developing bones as they start touching the face. The legs are beginning to move too. Your fetus can now open and close its fists and mouth, and the toes and fingers are unwebbed. Fingernails and toenails are starting to develop as the external ears avail. Your baby is about 3/4 inch long.

10 Weeks

fetal development

The developing baby is now officially a fetus. All organs and extremities are present and will continue to mature to become functional. Some of the vital organs such as the brain, kidneys, liver, and intestines begin to function. The circulatory and urinary systems are working, and the liver is now producing bile. The fetus is practicing some breathing movements although the umbilical cord supplies the needed oxygen. As the eyes development progresses, the eyelids are fusing to protect them. The translucent skin is becoming more opaque as the genitals start to build, but you cannot predict the gender yet after an ultrasound. The baby is now almost 1 ¼ inches in length.

11 Weeks

Your baby is almost fully developed since all the major organs, and overall structure are already formed and beginning to take shape. The fetus can sigh, hiccup, stretch, suck a thumb and move his or her head. The head is still large to constitute half of the body.  At 2 inch length, the baby’s genitals start to take on their gender form – male or female.

12 Weeks

The baby’s face now resembles what human beings have. The limbs are progressing, and they look like those of a newborn. Your baby’s posture becomes less curled and more upright and may have a length of 3 inches from the head to heels. The weight at week 12 is about half an ounce.

It is also during this week that the baby seems more defined in the pictures in the baby’s development timeline. The heartbeat is more robust at this juncture than last week. Head growth begins to slow down for the rest of the body to catch up.

13 Weeks

Your baby’s face shows more distinctive features than before. The lips and nose are now entirely formed. More hiccup bouts are happening in a bid to strengthen the diaphragm, which is readying the respiratory system for outside environment breathing. Most of the time in this week, the baby is flexing the newly developed joints and muscles. Your baby can now produce hormones, absorb nutrients and filter waste. More functioning organs include the thyroid, pancreas, and gallbladder. The fetus is now producing urine, and the bone marrow is working on the immune system to be utilized after birth.

Fetal Development: the Second Trimester

14 Weeks

The baby’s facial muscles are developing and flexing which implies that you can see him or her making facial expressions if you had a visual. Other tiny organs, muscles, and nerves are now functioning. The intestines are embedding into the body as the liver and pancreas produce bile and insulin respectively. Overall length at week 14 is about 4 – 4 ½ inches and weighs about 1 ¾ ounces.

15 Weeks

With the lips and nose already formed, the taste buds are now kicking in as the baby’s vision continues to develop. The head is now growing lanugo – the first form of hair. During the 15th week, the fetus is about 5 inches in length and 2 ounces heavy. According to research, females show more mouth movements than males while in the uterus.

16 Weeks

This week’s height and weight projections are about 4 ½ inches and 2.8 ounces respectively. With such size, the baby can fit on your palm and has more room for growth. The bones are hardening, and the rapid growth may or may not make you feel the fetal movements. At week 16, he or she is probably playing with the umbilical cord and trying to suck a thumb.

17 Weeks

fetal development week by week

A light and soft lanugo layer cover the baby to form fingerprint-like patterns on the body. The baby’s skin is still thin, but brown fat deposits are progressing, and they will help the fetus generate heat. After a few weeks, the eyes will start moving behind the lids in a side-to-side motion. Your baby is almost the size of your palm’s breath, with the height being approximately six inches and has an average weight of four ounces. The fetus is now heavier than the placenta.

18 Weeks

The baby is developing teeth during this week. With the ears currently functioning, the fetus can listen to your heartbeat and maybe galvanize upon hearing a loud sound. If you shine a bright light on your belly, the eyes can detect the light. Even though her retinas are sealed, they are now light sensitive. There is still too much room, so the fetus will be flexing out by reclining, sucking a thumb, and performing somersaults. Overall weight is now about 8 inches and weighs about 6 ounces.

19 Weeks

Your baby now measures 8 ½ ounces and 6 inches for the weight and height respectively. If you are carrying a female, she now bears your future generation since the ovaries contain millions of ova. During this week, the fetus is busy trying to reach the sac’s membrane, playing with the cord, sucking thumbs and flexing the legs.

20 Weeks

On the skin, Vernix caseosa is forming – a white and creamy substance that is protecting the skin. It is also during this week that antibodies are moving from your body to the fetus. The baby’s sweat glands are forming, and she is now about 10 inches, weighing 9 ounces.

21 Weeks

The 21st week of the baby’s development timeline marks when the fetus starts to put on weight seriously. He or she is drinking much of the amniotic fluid and urinating on it. On the other hand, the liquid keeps refreshing after three hours. The eyelids and eyebrows are fully developed but still closed. Taste buds are also on the tongue.

Weight: about ¾ pounds

Height: 10 ½ inches

22 Weeks

Your baby is proliferating from this week onwards, and the brain is not left behind. Fingernails are still coming out, and the organs are now about their primary duties. There are lips in the mouth area, and the gums inside are growing canines and molars beneath. The baby now looks like a newborn.

Weight: almost 1 pound

Height: about 11 inches

23 Weeks

If your baby is male, the already formed testicles are moving to the groin. For the female case, the ovaries are good to go. The eyes are there, but they still need pigmentation. Body proportions resemble that of a newborn, but the fetus needs more growth and fattening. Your baby’s body proportions are similar to a newborn’s, but the fetus is still thin.

Weight: approximately 1 pound

Height: 11 ½ inches

24 Weeks

fetal development week by week

At week 24, the baby now has sleeping patterns. Lanugo is slowly fading away to give room for the growing hair. The skin is growing to become more translucent with every pigment deposit, but it is wrinkled due to rapid development that overpasses fat deposition.

Hand and footprints are generating, and the fetus is having hiccup bouts. He or she will respond to your movements and sound from the outside environment. If you give birth during this week, there is almost a 50% chance that the baby will survive.

Weight: about 1 ¼ pounds

Height: about 12 inches

25 Weeks

In the last milestone of baby development timeline, the weight will be double or triple. Now that the fetus has a sleeping schedule, he or she will be active and inactive depending on whether it’s sleeping or awake time. Apart from touching the feet, the baby can even make a fist. If your partner presses his ear on your belly, he can hear the heartbeat. The baby’s plugged nostrils now open up.

Weight: about 1 ½ pounds

Height: Slightly longer than 13 inches

26 Weeks

It is during this week that your baby’s hearing and vision abilities achieve a significant milestone. The lungs are also working to become fully developed. The eyes are opening up which means the fetus sight is almost complete. Regardless of your baby’s genetic composition, they all have blue eyes during the gestation period, and they will attain their color after birth. The cochlea and peripheral sensory organs in the ear are now finally built. As the weeks progress, the baby will be more sensitive to loud noise. Back in the lungs, the air sacs and alveoli will be available towards the end of this week. They will start to produce surfactant which prevents the tissue from sticking together and give room for inflation.

The fetus is also sleeping for extended periods especially when you are asleep too.

Weight: almost 2 pounds

Height: 14 inches

27 Weeks

fetal development

If your baby was born during this week, there is about an 85% chance that he or she will survive. The lungs level of progress is enough for the baby to breathe by itself but for the sake of temperature regulation, there’s need for an incubator.

The liver and immune system are catching up at week 27, but they are still weak for outer environment survival.

Weight: 2 pounds

Height: 14-15 inches

Fetal Development: the Third Trimester

28 Weeks

All the primary organs such as the liver, lungs, and brain are on their final touches towards full development. For the brain, its weight will have a 400-500% increase from now to birthday. The baby can now smell, taste and ‘cry’ since the eyes are producing tears.

The bones are still soft, but growth is also in progress in a bid to harden them. As fat continues to build up, swallowing and sucking activities are also increasing.

Weight: Slightly above 2 ½ pounds

Height: about 15 inches

29 Weeks

Apart from the fat addition, the eyelashes are also growing. As the brain continues to develop, it can now sense breathing patterns and also control the body temperature. The present brainwave activity leads to the speculation that the fetus can even dream. Adrenal glands are working to produce a chemical that the placenta aids in transforming to estriol – an estrogen form which researchers think it stimulates the production of prolactin, which in turn brings forth milk production.

If the fetus were born during this week, you would still breastfeed.

Weight: about 2 ½ pounds – same or above last week’s value

Height: 15-17 inches

30 Weeks

The brain is still gaining complexity, and the eyes can now react to light. Some researchers say that flashing a beam of light once in a while over your belly may stimulate development. From now on to birth, the rate of weight gain will be slower than before. If the baby has not yet turned to a head-down position, give him or her time to do so. The fetus is now floating in about one and a half pints of the amniotic fluid which implies there still room for movement. The upside-down position is known as cephalic.

31 Weeks

Your baby is now gaining more weight than height, but the rates vary depending on individuals. As the room for growth decreases, there are more baby kicks than before, resulting in about 10 per hour. Your health care provider may suggest keeping track of the punches so that you are aware when they decrease. Other care providers may avoid the idea of taking notes as long as it feels like the baby is active. If you note a decrease in the number of kicks, you can take a glass of juice and wait. If it does not liven up the baby, contact your physician.

Weight: 2 ½-3 ½ pounds

Height: 14-16 inches

32 Weeks

All the five senses are ready and working not to mention the commencing of the dream cycle. More hair is growing as lanugo disappears and the fetus is practicing on how to breathe. If you give birth during this week, the baby will survive without complications.

Weight: about 4 pounds

Height: about 17 inches

33 Weeks

The baby may now be in the cephalic position. In the next coming weeks, the fetus will descend to the pelvic area. Apart from preparing for delivery, going to that position enables the brain to gain more blood flow. The dark and quiet womb provides a perfect venue for this to happen. Also progressing in growth is the immune system which is still receiving antibodies from you.

Weight: about 4 ½ pounds

Height: about 17 inches

34 Weeks

fetal development week by week

More connections are still forming in the brain, and with the progress so far, the baby can learn while in the womb. While it develops, the baby will be sleeping most of the time. He or she may also be darting the eyes while asleep and maybe dreaming. Even after birth, the brain will continue to grow up to ¾ of an adult’s size.

More action is in the lungs at week 34 now that the baby has some deeper breaths every time they breathe in. An ultrasound will show a blinking baby when awake, and the pupils are reacting to light.

Weight: about 5 pounds

Height: approximately 18 inches

35 Weeks

This week shows more progress in fat gain which aids in regulating body temperature. The immune system is also getting stronger but still has a long way to go. After birth, your first milk is needed to keep boosting it to the required level not to mention the necessary vaccinations. Everything else is now ready, and with every passing hour, the baby becomes more prepared for birth.

Weight: more than 5 pounds

Height: 16-20 inches

36 Weeks

This week, there is nothing more apart from the continuing brain development and more weight.

Weight: about 6 pounds

Height: 20 ½ inches

37 Weeks

Your baby now looks like a newborn. He or she is straining to breathe – a reason for all those kicks after a short while. The intestines are still collecting meconium which is composed of lanugo, dead cells, and amniotic fluid. It will later be discharged as the first poop after delivery.

Just like most of the pregnancies, the baby is starting to move towards the delivery position.

Weight: 6-7 pounds

Height: 20-21 inches

38 Weeks

With more restriction to movement, the neurons are still connecting in the brain, and the nails have already grown to reach the same height as the finger and toes tips.

39 Weeks39 week pregnancy

As the baby prepares for birth, the brain is still building more connections and adding weight.

Weight: 6-10 pounds

Height: 17-23 inches

40 Weeks

Your pregnancy is now joining the full term edge. Congratulations for attaining this far. Anytime from now, labor bells will be ringing if not yet.

Full-term weight: about 8 pounds

Full-term length: about 21 inches


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