Down syndrome is the most common genetic abnormality in newborns and, contrary to popular belief, most newborns with the condition are born to women 35 years and younger. For this reason, new guidelines now recommend that women of all ages be screened for Down syndrome before the 20th week of pregnancy.

Today, screening tests are more sensitive and can be used earlier in pregnancy to determine a women's risk of having a child with Down syndrome. But it is important to know that these screening tests cannot make the diagnosis of Down syndrome. Rather, they indicate that the likelihood of the fetal disorder is high enough to consider proceeding with one of two diagnostic tests - amniocentesis or chorionic villus sampling (CVS). Both procedures provide similar information, and both pose risks to the unborn fetus.

It's important also to understand that the false-positive rate for these tests is about 5 percent. This means that approximately 5 out of 100 women might undergo an unnecessary amniocentesis or CVS.

If screening tests indicate there is a high risk of Down syndrome, your doctor may recommend one of the following procedures to diagnose Down syndrome in your baby:

• Amniocentesis. In amniocentesis, a needle is directed through the abdominal wall of the mother into the amniotic sac that surrounds the fetus. Examination of cells in the fluid that is withdrawn through the needle can identify the characteristic chromosomal abnormalities in Down syndrome and in other disorders. Amniocentesis is usually carried out between the 15th and 17th weeks of gestation. The failure rate is higher and there are more complications if the procedure is done earlier. Waiting longer can also produce problems, however, if the parents are considering terminating the pregnancy based upon abnormal results, which may not be available for about a week after the procedure.
• Chorionic villus sampling (CVA). This test involves insertion of a needle into the placenta to obtain a small sample of tissue for chromosomal or DNA analysis. The needle is inserted either through the abdominal wall or through the uterine cervix. CVA is performed after the 10th week of pregnancy and is safer than early amniocentesis.

Both amniocentesis and CVA are invasive procedures associated with several risks, particularly miscarriage and damage to the fetus. In a recent study, approximately one out of every 1,600 amniocentesis procedures resulted in the death of the fetus.

The purpose of the screening tests is to minimize the number of these invasive procedures so that they are carried out only in those pregnancies where there is a high risk of Down syndrome (and other chromosomal abnormalities). On the other hand, the screening tests can miss the presence of Down syndrome in as many as 15 to 20 percent of pregnancies.

Therefore, women may opt to go directly for amniocentesis or CVA if they are at especially high risk for a fetus with a chromosomal abnormality. High risk could be signaled by a family history of genetic abnormalities, a child born with chromosomal abnormalities in a prior pregnancy, or the advanced age of the mother.

A woman may choose not to undergo these invasive procedures regardless of the results of the screening tests if, for example, she is unwilling to terminate her pregnancy. Even so, since Down syndrome children may need special care at the time of delivery, anticipating a Down syndrome offspring might affect your choice of hospital and obstetrician, who would then be prepared to deal with any problems during birth.

The new screening guidelines can help to limit the birth of Down syndrome babies, but increasing the likelihood of a normal offspring has become much more complicated. All pregnant women need to seek early prenatal care that includes thoughtful counseling on the options available to them. Physicians need to be aware of which screening procedures are available in their area and be prepared to offer appropriate information and advice, or to refer their pregnant patients to a genetics counseling professional or maternal-fetal medicine specialist.