Many birth defects can be diagnosed before birth with tests. Chromosome problems such as Down syndrome can be diagnosed before birth by looking at cells in the amniotic fluid or from the placenta. Or they can be found by looking at the baby’s DNA in the mother’s blood (noninvasive prenatal screening). These tests are very accurate.
Fetal ultrasound during pregnancy can also show the possibility of certain birth defects. But ultrasound is not 100% accurate. Some babies with birth defects may look the same on ultrasound as those without problems.
Tests that can be done while a baby is in the uterus include the following.
This blood test measures the levels of a protein called AFP (alpha-fetoprotein). This protein is released by the baby’s liver and is found in the mother's blood. AFP is sometimes called maternal serum AFP (MSAFP). AFP screening may be one part of a 2-, 3-, or 4-part screening. It's often called a multiple marker screen. The other parts are:
- Human chorionic gonadotropin (hCG). This is a hormone released by some cells in the placenta. High hCG levels may mean that the baby has Down syndrome. This condition is a chromosome problem. It causes learning problems and some physical changes.
- Estriol. This is a hormone made by the placenta, and by the baby’s liver and adrenal glands. Low levels may mean the baby has Down syndrome.
- Inhibin. This is a hormone made by the placenta. Abnormal levels may mean an increased risk for Down syndrome.
Nuchal translucency screening
This is an ultrasound test often done late in the first trimester. Thickening of the area at the back of the baby’s neck may mean an increased risk for Down syndrome or other chromosome problems.
This test takes a small sample of the amniotic fluid that surrounds the baby. The sample is used to diagnose chromosome problems and open neural tube defects such as spina bifida. Testing can be done for other genetic defects and disorders. This depends on family history and lab testing available at the time of the test. The American College of Obstetricians and Gynecologists advises amniocentesis around 15 to 20 weeks of pregnancy for women who are at higher risk for chromosome problems. These include women who are older than 35 at delivery. It also includes women who have had an abnormal blood test that means they are at higher risk for a chromosome problem or neural tube defect such as spina bifida. In some cases, amniocentesis may be done as early as 14 weeks.
Chorionic villus sampling (CVS)
This test takes a sample of some tissue from the placenta. This tissue has the same genetic material as the baby. It can be tested for chromosome problems and some other genetic problems. This depends on family history and lab testing available at the time of the test. Unlike amniocentesis, CVS does not give information on neural tube defects such as spina bifida. Women who have the CVS test also need a follow-up blood test between 16 and 18 weeks of pregnancy. This blood test is to screen for neural tube defects.
This is a test that uses sound waves to create an image of the internal organs. Some physical birth defects can be found with ultrasound.
Noninvasive prenatal screening
This is a test that looks at the mother's blood. The lab looks for genetic material called DNA that comes from the placenta. This DNA is the same as the baby’s DNA. The lab looks for chromosome problems such as trisomy 13, trisomy 18, and trisomy 21.
Sometimes birth defects are not diagnosed until after birth. A baby may be diagnosed with a physical exam by a healthcare provider. The healthcare provider may also take a blood sample. This is looked at in a lab to find chromosome problems. This is important so the parents know the risk for that birth defect in future pregnancies.