Prenatal Testing for Birth Defects

In the United States, about one in 33 babies are born with a birth defect, many of which can be detected while the baby is in the womb. For some parents, testing for birth defects begins even BEFORE they are pregnant! Known as genetic testing, or carrier screening, pre-pregnancy testing involves taking blood samples from both prospective parents. The samples are then screened for abnormal genes, which could potentially lead to a genetic birth defect, like cystic fibrosis or sickle cell anemia. If gene abnormalities ARE detected, the parents may meet with a genetic counselor to discuss their options, which range from finding out AFTER their baby's birth if there is a problem, using In Vitro Fertilization, or IVF, to ensure that the egg and sperm are free of the potential genetic anomaly, to getting pregnant using a donor sperm or egg. Because many pregnancies are unplanned, some parents choose to undergo genetic testing in the early weeks after conception. But even if they skip this step, most DO consent to a screening called the Nuchal Translucency Test, or NT. An NT is an ultrasound-based test that is usually given between weeks 11 and 14 of a pregnancy. This screening measures a baby's risk for developing chromosomal abnormalities like Down syndrome and trisomy 18, and certain major heart problems. During an NT, the clear space at the back of a fetus's neck, the nuchal fold, is measured. Babies with chromosomal abnormalities tend to have thicker nuchal folds, which appears as more translucent space in this area. Like all screenings, an NT does NOT definitely say if there is a problem. Instead, a Nuchal Translucency Screening, usually done in conjunctions with a blood test, which is sometimes referred to as a sequential screen shows, with 90 to 94 percent accuracy, how LIKELY it is that a genetic abnormality is present. If the risk is high, there are several options available. For one, she can wait until early in her second trimester to have a multiple marker screening. This blood test looks for three substances, which may indicate the presence of a chromosomal birth defect or a neural tube defect, like spina bifida. Like an NT, the multiple marker screening can predict the likelihood of an abnormality with about 70 to 85 percent accuracy. To know for sure if there's a problem, it's necessary to undergo one of two diagnostic tests: chorionic villus sampling, or an amniocentesis. Chorionic villus sampling, or CVS, is performed in the first trimester, between the 11th and 13th week of pregnancy. During the procedure, a doctor obtains a sample of tiny projections on the placenta called chorionic villi. The extraction is performed through the cervix, using a catheter or through the abdomen, using a long, thin needle. The placenta sample is then sent to a lab, where screening can tell whether a baby has a chromosomal abnormality or a genetic disorder. But CVS does NOT test for neural tube defects and the rate for miscarriage is higher with this procedure than it is with amniocentesis, a more thorough diagnostic test given in the second trimester. Amniocentesis is performed during weeks 16 to 20 of a pregnancy. The test involves inserting a long, thin needle into the abdomen to extract a sample of a baby's amniotic fluid. This fluid is then analyzed for the presence of chromosomal defects. The average rate of miscarriage for CVS is about 1 in 200, and the rate of miscarriage for amniocentesis ranges from 1 in 200 to over 1 in 1,000. MUCH depends on the skill of the person performing the test, so be SURE to research the doctor working with you! Because each procedure is more than 99 percent effective at testing for birth defects, many couples decide that the risk is worth it. If either a chorionic villus sampling or amniocentesis comes back positive, it's wise to seek the knowledge of a genetic counselor. Even parents with no intention of terminating a pregnancy can benefit from learning, as much as they can about their baby-to-be's condition. So remember: If you're pregnant or thinking about becoming pregnant, talk to your doctor about screening for genetic birth defects.