Amniocentesis involves the withdrawal of amniotic fluid through a fine needle which has been inserted through the mother's abdomen into the amniotic sac (bag of water) surrounding the baby. This fluid contains cells shed from the baby's skin, lungs, and urinary tract. These cells and fluid can be analyzed for certain types of problems with the baby.

The most common study performed on the fluid is a chromosomal analysis, which not only identifies Down's Syndrome, but other chromosomal abnormalities as well. Neural tube defects, or open areas of the skull or spine, can also be detected by amniocentesis. Later in pregnancy, amniocentesis can be used to check whether or not the baby's lungs are mature. Amniocentesis is also done for several other less common reasons, such as too much amniotic fluid or to check a baby's well-being if a problem is suspected.

The amniocentesis procedure is done under ultrasound guidance to avoid critical structures of the baby and mother. It is not unusual to delay the amniocentesis if an adequately safe insertion site is not found. Local anesthetic is usually injected to numb the skin's surface. After the needle is inserted, about 2 tablespoons of fluid are withdrawn through a syringe.  After the procedure, heavy lifting and strenuous activity is discouraged for about 24 hours.  Usually patients feel more comfortable if someone drives them home.

The decision to have an amniocentesis should not be taken lightly. The risk of losing the pregnancy from an amniocentesis done in the fourth to sixth month of pregnancy is about 1 in 200. The usefulness of the information gained from the amniocentesis and how that information will be used should be considered carefully. Later in pregnancy, pregnancy loss from amniocentesis is minimal, but premature labor and delivery occasionally occur.