Depending on the birth defect, it can either be diagnosed during pregnancy (as in the case of chromosomal abnormalities such as Down syndrome) or after the baby is born. Fetal ultrasound may give information on some birth defects while the baby is in the womb, but it is not 100 percent accurate. A chromosomal analysis, whether performed on a blood sample or cells from amniotic fluid or the placenta, is more than 99.9 percent accurate. Talk to your doctor when deciding if testing is right for you.
Screening for birth defects while the baby is in utero may include the following tests:
- Alpha-fetoprotein: This test measures the level of alpha-fetoprotein, a protein released by the fetal liver and found in the mother’s blood. This test is often part of a multiple marker screen, which tests for hormones whose levels may indicate certain birth defects.
- Nuchal translucency screening: This is an ultrasound test performed late in the first semester, which may indicate an increased risk for Down syndrome or other chromosomal problems.
- Chorionic villus sampling (CVS): This is a prenatal test that involves taking a sample of placental tissue and testing it for chromosomal abnormalities and other genetic problems.
- Amniocentesis: A small sample of amniotic fluid is taken to test for chromosomal disorders and open neural tube defects such as spina bifida.
- Ultrasound: Ultrasounds use high-frequency sound waves to create an image of internal organs, and can detect many birth defects.
Prenatal screening for birth defects involves what are called screening tests, which are conducted to see if a woman or her baby may have problems. These tests are not perfect: They can sometimes give abnormal results even when the there is nothing wrong with the baby, or can indicate that everything is fine when it is not.
First Trimester Tests
First trimester tests involve a combination of tests completed between the 11th and 13th weeks of pregnancy. These tests are used to look for certain birth defects related to the fetal heart or chromosomal disorders such as Down syndrome. These tests include a maternal blood test and an ultrasound.
Second Semester Tests
Second trimester tests are conducted between the 15th and 20th weeks of pregnancy. They involve a maternal serum screen and a comprehensive ultrasound evaluation of the baby, which looks for structural anomalies.