What Is Amniocentesis?

Amniocentesis is a procedure performed during pregnancy in which amniotic fluid is withdrawn from a woman's uterus to test for certain problems in the fetus, such as genetic defects, fetal infections, fetal lung immaturity, or Rh sensitization. The word "amniocentesis" literally means "puncture of the amnion," the fluid-filled sac that encloses the fetus during pregnancy.

Doctors often recommend that women over 35 receive an amniocentesis screening, since there is a risk of Trisomy 21, or Down syndrome, in babies born to older women. Women who have a family history of genetic defects may undergo amniocentesis, as will women who want to clarify or confirm abnormal test results from other prenatal screenings. Amniocentesis is also sometimes used when women are carrying a baby with a conflicting blood type, to ensure that the fetus is in good health.

Amniocentesis is used to detect:

• Nearly all chromosomal disorders, including Down syndrome, trisomy 13, trisomy 18, and sex chromosome abnormalities (such as Turner syndrome and Klinefelter syndrome). Amniocentesis is more than 99 percent accurate in diagnosing these conditions.

• Several hundred genetic disorders, such as cystic fibrosis, sickle cell disease, and Tay-Sachs disease. The test is not used to look for all of them, but if your baby is at increased risk for one or more of these disorders, in most cases amniocentesis can usually tell you whether he has the disease.

• Neural tube defects such as spina bifida and anencephaly.