Screening for Down syndrome is performed in the first trimester, especially at 11-14 weeks of pregnancy, by measuring nuchal translucency combined with double test and NIPT test.
Down syndrome (trisomy 21) is the most common chromosomal abnormality. People with this syndrome often have mild, moderate or severe mental and cognitive development delays.
Down syndrome screening is a method of identifying pregnancies that are likely to have Down syndrome. If the fetus is in a high-risk group, pregnant women can choose procedures to accurately diagnose the disease such as amniocentesis, chorionic villus biopsy, umbilical cord blood collection…
According to Dr. Nguyen Hoang Long, Deputy Director of the Fetal Medicine Center, Tam Anh General Hospital, Ho Chi Minh City, the goal of Down syndrome screening is to provide early information to pregnant women and their families about the risk. fetus with Down syndrome. From there, the family can make appropriate decisions, based on conditions, circumstances, religion, and economics. At the same time, families prepare psychologically and learn about medical and community support for children with Down syndrome, helping them develop their full potential and improve their future lives.
Pregnant women have blood taken for testing at Tam Anh General Clinic, District 7. Illustration photo: Tam Anh
Combined test
Combining ultrasound to measure nuchal translucency and double test (testing two indicators of PAPP-A and free beta-hCG) is performed from 11 weeks to 13 weeks and 6 days, called a combined test. Nuchal translucency is a layer of clear fluid located behind the fetal neck. When there is a lot of fluid accumulation (thick nuchal translucency), it is associated with genetic syndromes, gene abnormalities, congenital heart defects, and structural abnormalities. According to Dr. Long, nuchal translucency can be measured by abdominal or vaginal ultrasound, with the same results. Accuracy depends on the experience and skill of the sonographer.
Biochemical index PAPP-A and free beta-hCG in maternal blood. PAPP-A levels increase with gestational age until late pregnancy. A decrease in this index is often related to abnormal placental function, warning of abnormal chromosome numbers 21, 18, 13 or intrauterine growth retardation, gestational hypertension, preeclampsia, and premature birth.
Free beta-hCG is a component of the hormone HCG, which is secreted during pregnancy. The hCG index in a pregnant woman’s serum peaks at week 10, gradually decreases until week 20, and remains unchanged until the end of pregnancy. High levels of hCG, especially free beta-hCG, are a sign that the fetus has Down syndrome. However, the concentration of these two hormones is affected by gestational age, maternal characteristics such as race, weight, smoking status, assisted reproductive measures…
Dr. Long said the combined test can detect about 90% of the risk of the fetus having Down syndrome, with a 5% false positive rate. Currently, the combined test combined with nasal bone index, flow through the tricuspid valve and venous duct can increase the accuracy rate to 93-96%, reducing the false positive rate to 2.5%. However, this method is not popular and is mainly performed at medical facilities with full modern equipment.
NIPT
According to Dr. Long, non-invasive prenatal testing (NIPT) can screen for Down syndrome after 10 weeks of pregnancy, with 99.7% accuracy, with a false positive rate of 0.04%. Therefore, pregnant women can reduce the number of invasive procedures and reduce anxiety. “NIPT has higher value than the combined test in screening for Down syndrome,” Dr. Long said, adding that however this is still a screening test, it cannot replace invasive procedures in diagnosis. Down syndrome, should not be used to advise on termination of pregnancy.
This technique also cannot replace the role of ultrasound in the first trimester of pregnancy (11-14 weeks). Ultrasound at this stage provides a lot of important information such as determining gestational age, number of fetuses, early detection of major abnormalities, other chromosomal abnormalities, screening for preeclampsia, and nuchal translucency. Increased nuchal translucency is also associated with cardiac abnormalities, genetics or increased rates of intrauterine arrest.