Human Genetics Laboratory

Amniocentesis is a procedure performed by a physician to obtain a small amount of the fluid (amniotic fluid) surrounding the baby as it develops. This fluid contains cells shed from the skin of the fetus. These cells can be cultured (grown) and analyzed in the laboratory and appropriate tests can be run including examining the chromosomes (for conditions such as Down syndrome) and specialized biochemical and DNA tests in families with a history of certain genetic disorders (such as Tay Sachs or Cystic Fibrosis). The fluid itself can be analyzed for the amount of specific chemicals such as AFP. Results of chromosome analysis are generally available in 10-14 days while the time to receive results of other tests may vary.

Routinely, amniocentesis is performed 15-18 weeks after the woman's last menstrual period. It can be done later, if necessary. "Early" amniocentesis can be performed at 12-14 weeks gestation. 

If twins are present, a sample must be taken from the sac surrounding each baby. Although the ultrasound exam may take some time, the procedure itself takes only a few minutes. When asked, most women mention minimal discomfort during the procedure, thus anesthesia is not necessary. In the event the first attempt is unsuccessful, a second attempt may be made. In the unlikely event that a fluid sample is not obtained, the procedure will be rescheduled, if desired, usually within a week. 

An ultrasound scan of the fetus is done prior to amniocentesis to assure the baby is living, confirm the correct gestational age, determine the number of babies, and locate the optimal site for the needle insertion. A full bladder is required for the procedure. The mother's abdomen is cleansed with antiseptic. Using sterile technique, ultrasound guidance is used to direct a thin needle through the mother's abdomen into the uterus where a small amount of fluid is withdrawn. In most cases this fluid is replaced by the fetus within 24 hours. Great care is taken to avoid touching the baby with the needle.