Down syndrome, also known as trisomy 21, is a genetic disorder caused by an extra copy of chromosome 21. This extra genetic material leads to intellectual disability and characteristic physical features in babies born with Down syndrome. As a pregnant woman, you may be wondering how early in pregnancy it’s possible to detect Down syndrome and what the options are for screening and diagnostic testing.
First Trimester Screening
There are two main screening options in the first trimester to assess the risk for Down syndrome:
- Nuchal translucency (NT) scan: This ultrasound is done between 11-14 weeks and measures the clear space at the back of the baby’s neck. More space than normal may indicate Down syndrome.
- Blood tests: A sample of the mother’s blood is tested for levels of two proteins – pregnancy-associated plasma protein A (PAPP-A) and human chorionic gonadotropin (hCG). Abnormal levels may indicate Down syndrome.
These screens provide a risk assessment, not a definitive diagnosis. The results are usually expressed as a ratio (such as 1 in 100). A higher ratio indicates a higher risk for Down syndrome. If the risk is high, further diagnostic testing may be recommended.
Nuchal Translucency Scan
The nuchal translucency (NT) scan is done between 11-14 weeks of pregnancy and uses ultrasound to measure the clear space in the tissue at the back of the baby’s neck. Known as the nuchal fold, extra fluid here may indicate chromosomal abnormalities like Down syndrome.
Doctors will also look for other markers during the NT scan that may suggest an increased risk of Down syndrome, such as a bright dot in the heart called an echogenic intracardiac focus.
The NT measurement is then combined with the mother’s age to provide a risk assessment for Down syndrome. The results are usually available within a few days.
First Trimester Blood Tests
Two proteins produced during pregnancy are measured from a blood sample from the mother:
- Pregnancy-associated plasma protein A (PAPP-A): This protein is made by the developing placenta and helps nourish the growing baby early in pregnancy. Lower levels may indicate Down syndrome.
- Human chorionic gonadotropin (hCG): This is a hormone present at high levels in early pregnancy. Higher levels may indicate Down syndrome.
The levels of these proteins, combined with mother’s age, are used to calculate the risk of Down syndrome. Results are usually available within a week.
Combined First Trimester Screening
Many providers offer combined first trimester screening, which includes both the NT scan and the blood tests. Combining these results provides a more accurate risk assessment for Down syndrome than either test alone. The combined results have a detection rate of approximately 90% with a false positive rate of 5%.
When are results available?
If you opt for first trimester screening, the results are usually available by the time you reach 14 weeks of pregnancy.
Second Trimester Screening
Second trimester screening for Down syndrome typically includes:
- Quad screen: A blood test that measures four pregnancy-related proteins.
- Anatomy scan: An ultrasound that looks for physical abnormalities associated with Down syndrome.
These screens are done later in pregnancy than first trimester screening – usually around 15-22 weeks. The advantage of second trimester screening is that it’s an option even if you didn’t get first trimester screening done.
Quad Screen
The quad screen measures the levels of four proteins in your blood between 15-22 weeks of pregnancy:
- Alpha-fetoprotein (AFP)
- Estriol
- Human chorionic gonadotropin (hCG)
- Inhibin A
Abnormal levels of these proteins may indicate an increased risk for Down syndrome. The quad screen can detect around 80% of Down syndrome pregnancies with a false positive rate of 5%.
Anatomy Scan
During this ultrasound, usually done between 18-22 weeks, the doctor looks carefully at the development of the baby’s bones, organs, and tissues. Babies with Down syndrome often have detectable physical differences such as:
- Shorter limbs
- Flattened facial profile
- Single crease across the palm of the hand
- White spots in the iris of the eye called Brushfield spots
- Small or absent nasal bone
Around 50% of Down syndrome pregnancies may be identified with an anatomy scan alone. But the detection rate improves to 80% when combined with the quad screen.
When are results available?
If you opt for second trimester screening, your Down syndrome risk assessment will be available sometime between 15-22 weeks. This is later than first trimester screening, but can still provide valuable information to guide further testing if the results are abnormal.
Diagnostic Testing
Screening tests estimate the risk or chance that a pregnancy is affected by Down syndrome but cannot provide a definitive diagnosis. There are two main diagnostic tests available that can detect Down syndrome during pregnancy with nearly 100% accuracy:
- Chorionic villus sampling (CVS): A small sample of the placenta is tested and results are available within 1-2 weeks.
- Amniocentesis: A sample of the amniotic fluid is tested and results are available within 2-3 weeks.
These tests are invasive and carry a small risk of miscarriage. For this reason, they are usually only recommended when a screening test indicates a high risk for Down syndrome.
Chorionic Villus Sampling
CVS involves using a thin tube or needle guided by ultrasound to take a small sample of cells from the placenta. This is called the chorionic villi and contains the same genetic material as the fetus. The cells are then tested for extra or missing chromosomes.
CVS can be done earlier than amniocentesis, from 10-13 weeks of pregnancy onwards. Results are available within 7-10 days.
The miscarriage rate after CVS is estimated to be about 1 in 500 to 1 in 1000.
Amniocentesis
A sample of the amniotic fluid surrounding the baby is taken using a thin needle guided by ultrasound. Fetal cells shed from the skin, urine, and respiratory tract are present in this fluid and can be tested for chromosomal abnormalities.
Amniocentesis is done from 15 weeks of pregnancy onwards. The full results are available within 10-14 days.
The risk of miscarriage after amniocentesis is estimated to be about 1 in 400 to 1 in 1000.
Non-Invasive Prenatal Testing (NIPT)
A newer option for Down syndrome screening is non-invasive prenatal testing (NIPT). This is a blood test that can screen for Down syndrome with greater than 99% accuracy by detecting fetal DNA in the mother’s bloodstream.
NIPT can be done from 10 weeks of pregnancy onwards with results in approximately 1 week. Because it’s non-invasive, there is no risk of miscarriage. NIPT is not diagnostic, however, so a positive screen would need to be confirmed with diagnostic testing.
Limitations of NIPT include the possibility of false positives, false negatives, and the inability to detect other chromosomal abnormalities as accurately as a diagnostic test.
Options for Screening and Diagnostic Testing
This table summarizes the main options for Down syndrome screening and diagnostic testing during pregnancy:
| Test | Timing | What’s involved | Accuracy | Risks |
|---|---|---|---|---|
| First trimester screening | 11-14 weeks | Blood test and/or nuchal translucency ultrasound | 80-90% detection rate 5% false positive rate |
No risk |
| Second trimester screening | 15-22 weeks | Quad screen blood test and anatomy ultrasound | 80% detection rate 5% false positive rate |
No risk |
| Chorionic villus sampling (CVS) | 10-13 weeks | Removing a sample of placenta cells | ~100% accurate | 1 in 500 to 1 in 1000 miscarriage risk |
| Amniocentesis | After 15 weeks | Removing a sample of amniotic fluid | ~100% accurate | 1 in 400 to 1 in 1000 miscarriage risk |
| Non-invasive prenatal testing (NIPT) | 10 weeks onwards | Blood test | >99% detection rate Low false positive rate |
No risk |
Conclusion
There are screening options in both the first and second trimester to assess risk for Down syndrome. First trimester screening (combined blood tests and NT ultrasound) provides results early at 11-14 weeks. Second trimester screening can be done even if first trimester screening was missed.
Diagnostic testing with CVS or amniocentesis can definitively confirm Down syndrome but comes with a small risk of miscarriage. Non-invasive prenatal screening using cell-free fetal DNA offers highly accurate screening with no risk to pregnancy.
Talk to your health care provider about the options and recommended timing for prenatal screening for Down syndrome based on your individual risk factors and preferences. With the variety of screening and diagnostic tests now available, Down syndrome can often be detected with accuracy very early in pregnancy.