Amniocentesis: Why and How It’s Done

Amniocentesis is a diagnostic procedure in which a small amount of amniotic fluid is withdrawn from the amniotic sac, the membrane that surrounds the fetus in the uterus.

Why is amniocentesis done?

Amniocentesis and chorionic villus sampling can be used to diagnose certain fetal abnormalities early enough in a pregnancy for elective abortion to remain an option. But because these procedures carry a small risk of inducing a miscarriage or early labor, they are recommended only when there are good medical reasons for doing them such as when a woman is older than age 35 or  there is a family history of a chromosomal abnormality.

Amniotic fluid contains cells and chemicals from the fetus that can be analyzed to find fetal abnormalities, such as Down’s syndrome, which is a chromosomal abnormality. Amniocentesis can also help find other chromosomal abnormalities and disorders such as hemophilia in males, metabolic diseases such as Tay-Sachs disease, or developmental disorders such as spina bifida. It is also used to assess disorders of the fetus such as hemolytic disease and respiratory distress syndrome.

How is amniocentesis done?

Genetic amniocentesis is usually done between 16 and 18 weeks of gestation because before that time, there is generally not enough amniotic fluid. Ultrasound scanning is used to estimate the age and position of the fetus, the location of the placenta and where it is attached, and the amount of amniotic fluid in the amniotic sac. A long needle is inserted through the abdomen and wall of the uterus into the amniotic sac, avoiding the fetus and placenta. A syringe is attached to the needle and about 20 to 30 mL of fluid is removed.

Sometimes a local anesthetic is used. The mother can usually go home soon after the procedure is done but should rest for 24 hours.


The amniotic fluid may be analyzed biochemically and chromosome culture done to test for abnormalities of the fetus such as down syndrome and spina bifida.  Chromosome cultures take up to 4 weeks, so results may not be available until about 20 weeks gestation. The gender of the fetus is also determined by the chromosome analysis and the mother should let her doctor know if she wants to know the gender of the baby.


There’s a slight increase in the number of threatened miscarriage and early rupture of the membranes after the procedure. For these reasons, amniocentesis is usually recommended only for women over the age of 35 (who are more likely to have a child with Down’s syndrome) or when there are other compelling medical reasons, such as family history of a chromosomal abnormality.