Screening for Birth Defects
Screening tests can help assess the risk of having a baby with a birth defect. Some women choose to have screening tests depending on their age, medical history, previous pregnancies, family history or ethnic background. Most women, however, choose to have screening for reassurance.
Almost all children in the United States are born healthy. Out of 100 newborns, only two to three have major birth defects. For about 70% of babies born with birth defects, the cause is not known. In other cases, birth defects are inherited through genetics or caused by the mother being exposed to harmful agents or medications.
Most birth defects occur during the first three months of pregnancy. Some can be found before birth with screening tests. The most common birth defects found through screening tests include:
- Neural tube defects: incomplete closure of the fetal spine can result in spina bifida or anencephaly (absence of part of the skull)
- Abdominal wall defects: failure of the abdominal wall to close during development
- Heart defects: mild to severe abnormalities of heart shape or function
- Down syndrome: mental retardation, abnormal facial features, and other medical problems occur as a result of an extra chromosome 21 (also called trisomy 21)
- Trisomy 13 and 18: there is an extra chromosome 13 or 18, either of which causes severe mental retardation and severe birth defects
Types of Screening Tests
Screening tests are easy to perform and pose little risk to you or your fetus. Screening tests do not diagnose a birth defect, but they tell you if you are at higher risk of having a child with a birth defect than the average person. If the risk is high, you may choose to proceed with more invasive diagnostic testing. The following screening tests are available and routinely performed.
- First Trimester Screen: This test screens for Down syndrome and trisomies 13 and 18 and is done between 12 and 14 weeks. It includes a maternal blood test and an ultrasound. The ultrasound measures the fetus' neck fold thickness (nuchal translucency) and can take 30 minutes or more for the fetus to be in the correct position. First trimester screening is 85% accurate at detecting Down syndrome and 95% for trisomies 13 and 18.
- Alpha-fetoprotein (AFP): This is a maternal blood test for neural tube defects, such as spina bifida and anencephaly. It is done between 15 and 20 weeks. AFP detects neural tube defects 80% of the time. AFP is included in the quad screen.
- Quad Screen: This is a maternal blood test that measures four substances (AFP, estriol, human chorionic gonadotropin and inhibin-A), which are altered in Down syndrome, trisomies 13 and 18, and neural tube defects. The quad screen is done between 15 and 20 weeks and detects Down syndrome 80 percent of the time, trisomy 18 60-70 percent of the time and neural tube defects 80 percent of the time.
- Anatomy Ultrasound: All pregnant women are offered a comprehensive ultrasound between 20 and 22 weeks. This detailed survey of the fetus' organs and features can detect some birth defects.
Other Screening Tests
Several other screening tests are available to detect the following diseases:
- Cystic fibrosis is a severe respiratory and digestive disease most common among Caucasians.
- Thalassemia is a type of anemia more common in people of Mediterranean, Middle Eastern, African or Asian descent.
- Sickle cell anemia is another type of anemia common to people of African descent.
- Tay-Sachs is a disorder of fat metabolism that causes severe mental retardation, blindness and seizures and is more common in Ashkenazi Jews and French Canadians.
Your provider will discuss with you the available screening tests and what the results mean. If the results raise concerns about your pregnancy, additional diagnostic tests may be offered. Talk to your provider about which screenings are right for you.