beginning of the pregnancy to screen for possible genetic conditions. These groups include Ashkenazi Jews (for Tay-Sachs disease), African-Americans (for sickle cell anemia), Mediterranean people (for Beta Thalassemia), Southeast Asians and Chinese (for Alpha Thalassemia).

What it tells you. The tests vary according to the condition for which a woman is being screened. These initial blood tests will not provide conclusive information but rather screen for possible presence of the problem. If the test is positive, then a more complex procedure such as amniocentesis or chorionic villus sampling will be offered to determine actual genetic information about the baby.

Chorionic Villus Sampling (CVS)
10-12 weeks
What it measures. This test is usually offered to women who are at increased risk for genetic disorders. This might include a family history of a problem, abnormal screening tests or advanced maternal age. It involves taking a sample of placental tissue, called the "chorionic villi," which will allow analysis of the genetic makeup of the baby. The test is done under the guidance of ultrasound by a physician and can be done either trans-abdominal (through the stomach) or trans-cervical (through the cervix).

What it tells you. The reason some families choose this test over amniocentesis (below) is that it can be done at an earlier time in the pregnancy. Since the method allows for a direct look at the infant's chromosomes, it is very accurate - about 99% - for diagnosing abnormalities. CVS can increase a mother's risk for a miscarriage. One to two women out of 100 might have a miscarriage as a result of this procedure. This risk is even higher when a woman has a retroverted uterus (the uterus is tipped toward the back), and CVS is done through the cervix. In addition, recent studies have found an increase in a type of birth defect called limb reduction (shortened or missing fingers and toes) in infants whose mothers had CVS before the 10th week of pregnancy. For this reason, the procedure is not conducted until after 10 weeks.

Amniocentesis

Usually done at 15-18 weeks
What it measures. Like CVS, amniocentesis is offered to women at increased risk for genetic disorders. The test uses a sample of the amniotic fluid that surrounds the baby. It is performed by a physician who inserts a needle through a numbed area on the mother's stomach under ultrasound guidance. The amniotic fluid is then sampled and analyzed.

What it tells you. The fluid obtained contains fetal cells that can be examined for the chromosomes and genetic makeup. Like the CVS, it is a very accurate test with a 99.4%-100% rate of accuracy, diagnosing most chromosomal abnormalities. However, no form of screening or testing can diagnose 100% of genetic disorders, and some will be missed. While the test is considered safe by the National Institutes of Health, amniocentesis can pose a small increased risk of miscarriage (one out of every 200-400 women). However, when the procedure is done in the first trimester (before 12 weeks), the risk of miscarriage is three times higher, which is why the test is now done in the second trimester.