Amniocentesis is a prenatal test that allows the fetal medicine specialist to gather information about the baby's health from a sample of the fluid that surrounds the baby in the uterus. This fluid is called the amniotic fluid. Amniotic fluid contains skin cells shed from the baby and is used to diagnose chromosomal abnormalitis such as Down syndrome. Amniocentesis produces a karyotype – a picture of thebaby's chromosomes – to determine if there are any abnormalities. (In about 1 percent of cases, there's a problem with the specimen and the test doesn't yield a result.) Amniocentesis is usually done when a woman is between 16 and 20 weeks pregnant. Though all women have the option of having an amniocentesis, women for whom this test is recommended are often those at increased risk for genetic and chromosomal problems.The test is invasive and carries a small risk of miscarriage (0.01%)