Prental

Prenatal Testing

Prenatal testing to detect chromosomal defects and inherited genetic disorders has been widely available for women with high-risk pregnancies since the 1970s. These days nearly every pregnant woman in the U.S. has a preliminary maternal serum screening test performed.

The serum test, a simple blood test, is usually performed between week 15 and week 18 of the pregnancy (as measured from the start of the most recent menstrual period). The test evaluates your risk of having a baby with:

1. An open neural tube defect. When the neural tube fails to close, the baby is born with an opening in the head (anencephaly) or spinal cord (spina bifida). Babies with anencephaly are stillborn or die soon after birth; those with spina bifida need surgery and may be paralyzed.
2. Down syndrome (also called Trisomy 21). An extra copy of chromosome 21 causes Down syndrome, characterized by mental retardation, certain facial features and sometimes heart defects.
3. Trisomy 18. An extra copy of chromosome 18 causes this syndrome, which usually proves fatal during the first year of the baby's life and is associated with severe mental retardation.

The blood screen does not look directly at genetic material but instead measures three substances-alpha-fetoprotein, unconjugated estriol and human chorionic gonadotropin-to determine whether you are at increased risk of having a baby with one of these disorders. A key fact to remember is that this test does not diagnose the disorders-it only screens for the substances. Further testing is always suggested to make a diagnosis.

Indeed, most of the time, the fetus is not affected with the disorder even if the screening result is abnormal. According to Genzyme Genetics, a genetics laboratory based in Framingham, Mass, out of 1,000 serum screening tests, 25 will suggest an increased risk for open neural tube defects, but only one or two of the fetuses will have such a defect. Likewise 70 out of 1,000 will test positive for increased risk of Down syndrome, but only one or two fetuses will actually have the condition.

Most labs offer the standard "triple marker" serum screening test, which picks up about 70 percent of Down syndrome cases among women under 35, and 85 to 90 percent in women 35 and older. A recently introduced four-marker test incorporating a measure for a chemical called inhibin-A improves detection rates for Down syndrome by five to 10 percentage points. Some centers also offer first trimester screening to determine if a woman is at increased risk for having a baby with Down syndrome or trisomy 18. The test has two parts and both should both be performed between the 10th and 13 weeks of gestation. One part tests levels of maternal serum free beta-human chorionic gonadotropin (beta-hCG) and pregnancy-associated plasma protein-A (PAPP-A) in the mother's blood. The other measures the nuchal thickening (measure of fetal neck thickness) through a specialized ultrasound. This test detects more than 90 percent of fetuses with Down syndrome and approximately 97 percent with Trisomy 18. However, it is a screening test and still requires a confirmatory chorionic villous sampling or amniocentesis to make the definite diagnosis.

A screening test does not diagnose a problem. It should not be used to make either treatment decisions or decisions to terminate a pregnancy, but rather should be used to determine appropriate next steps.