First Trimester Screening
First trimester combined screening combines a nuchal translucency ultrasound with a maternal blood test to assess risk for chromosomal conditions. It's typically done at 11–14 weeks.
Typically performed: Weeks 11–14
The Two Components
Combined first trimester screening includes:
- Nuchal translucency (NT) ultrasound: Measures the fluid-filled space at the back of the baby's neck. Larger NT measurements are associated with higher risk for chromosomal abnormalities and some heart defects.
- Blood test: Measures two hormones — free beta-hCG and PAPP-A. Abnormal levels, combined with NT, age, and other factors, calculate a risk score.
Understanding Your Results
Results are expressed as a risk ratio (e.g., 1:2500 for Down syndrome). A "screen positive" result (usually above a threshold like 1:150 or 1:300) indicates increased risk and warrants follow-up with NIPT or diagnostic testing. A negative result significantly reduces — but doesn't eliminate — risk.
What It Doesn't Screen For
This screen does not detect all chromosomal conditions, structural abnormalities, or genetic disorders. The anatomy scan at 20 weeks is needed to assess physical development. NIPT has largely replaced combined first trimester screening at many practices due to higher sensitivity.
Frequently Asked Questions
No — it's a standard abdominal ultrasound. The technician may also do a transvaginal ultrasound for better imaging at early gestational ages.
An NT measurement of 3.5mm or more is considered elevated and warrants further investigation. It can be associated with chromosomal conditions, heart defects, or genetic syndromes — or can be a normal variation.
All prenatal screening is optional. You have the right to decline any screening or diagnostic test. Discuss what information would be useful to you and what you would do with results before testing.