Amniocentesis is a prenatal test in which a small amount of amniotic fluid is removed from the sac surrounding the fetus for testing. The sample of amniotic fluid (less than one ounce) is removed through a fine needle inserted into the uterus through the abdomen, under ultrasound guidance.

Why it's done

Amniocentesis can be done for various reasons:
1. Genetic testing. Genetic amniocentesis involves taking a sample of amniotic fluid and testing it for certain conditions, such as Down syndrome.
2. Fetal lung testing. Fetal lung maturity testing involves taking a sample of amniotic fluid and testing it to determine whether the baby's lungs are mature enough for birth.
3. Diagnosis of fetal infection. Occasionally, amniocentesis is used to evaluate a baby for infection or other illness. The procedure also can be done to evaluate the severity of anemia in babies who have Rh sensitization - an uncommon condition in which a mother's immune system produces antibodies against a specific protein on the surface of the baby's blood cells.
4. Treatment. If you accumulate too much amniotic fluid during pregnancy (polyhydramnios), amniocentesis might be done to drain excess amniotic fluid from your uterus