Prenatal Screening for Down SyndromeWomen over age 35 have an increased risk of giving birth to babies with Down syndrome and other chromosomal conditions. In addition, women with a previous child with a genetic birth defect (including chromosomal) or a family history of a genetic disorder, are also at risk. Your doctor may recommend that you meet with a genetic counselor, as well as order diagnostic tests to to assess your baby’s health.

There are several prenatal diagnostic tests that can be performed to determine the occurrence of Down syndrome. These tests include amniocentesis, chorionic villus sampling (CVS), and percutaneous umbilical blood sampling (PUBS).

Amniocentesis, the removal and analysis of a small sample of fetal cells from the amniotic fluid, is widely available and involves a lower risk of miscarriage than chorionic villus sampling. However, amniocentesis cannot be done until the 14th to 18th week of pregnancy, and it usually takes additional time to determine whether the cells contain extra material from chromosome 21.

Chorionic Villus Sampling, conducted at 9 to 11 weeks of pregnancy, involves extracting a tiny amount of chorionic villi tissue extensions(using and ultrasound as a guide) that will eventually develop into a placenta. The tissue can be tested for the presence of extra material from chromosome 21. The villi can be obtained through the pregnant woman's abdomen or cervix. This type of sampling carries a 1-2% risk of miscarriage.

Percutaneous Umbilical Blood Sampling or PUBS, is the most accurate method and can be used to confirm the results of CVS or amniocentesis. However, PUBS cannot be performed until later in the pregnancy, during the 18th to 22nd weeks, and carries the greatest risk of miscarriage.