Amniocentesis Procedure

By Bowers, Nancy BSN, RN, MPH

(Amniotic Fluid Analysis)

Procedure Overview

What is amniocentesis?

Amniocentesis is a procedure used to obtain a small sample of the amniotic fluid that surrounds the fetus during pregnancy. Amniotic fluid is a clear, pale yellow fluid made by the fetus. The fluid protects the fetus from injury and helps to regulate the temperature of the fetus.

In addition to various enzymes, proteins, hormones, and other substances, the amniotic fluid contains cells shed by the fetus. These cells contain genetic information that can be used to diagnose chromosomal disorders and open neural tube defects (ONTDs), such as spina bifida. Testing may be available for other genetic defects and metabolic disorders depending on the family history and availability of lab testing at the time of the procedure.

Amniotic fluid also contains other substances that provide information about certain conditions. An amniocentesis may be performed in late pregnancy to check fetal well-being and diagnose fetal conditions, such as Rh disease (complications resulting from incompatibility between the blood types of the mother and baby) and infection. An amniocentesis may be performed to check for fetal lung maturity if a baby is expected to be born prematurely. Specific substances present in the amniotic fluid can be measured or tested for these conditions.

The fluid is sent to a genetics lab so that the cells can grow and be analyzed. Results are usually available in about 10 days to two weeks, depending on the lab.

An amniocentesis is generally offered to women between the 15th and 20th weeks of pregnancy who are at increased risk for chromosome abnormalities. This includes women who are over 35 years of age at delivery, or those who have had an abnormal maternal serum (blood) screening test indicating an increased risk for a chromosomal abnormality or neural tube defect.

Amniocentesis helps confirm a tentative diagnosis of an abnormality previously found with other testing. It may also find that a fetus does not have the abnormality that was suspected.

Another related procedure that may be used to diagnose genetic and chromosomal defects is chorionic villus sampling (CVS). Please see this procedure for additional information.

Anatomy of the fetus in utero:

  • amniotic sac - a thin-walled sac that surrounds the fetus during pregnancy. The sac is filled with amniotic fluid (liquid made by the fetus) and the amnion (the membrane that covers the fetal side of the placenta), which protects the fetus from injury and helps to regulate the temperature of the fetus.

  • anus - the opening at the end of the anal canal

  • cervix - the lower part of the uterus that projects into the vagina. Made up of mostly fibrous tissue and muscle, the cervix is circular in shape.

  • fetus - an unborn baby from the eighth week after fertilization until birth

  • placenta - an organ, shaped like a flat cake that only grows during pregnancy and provides a metabolic interchange between the fetus and mother. (The fetus takes in oxygen, food, and other substances and eliminates carbon dioxide and other wastes.)

  • umbilical cord - a rope-like cord connecting the fetus to the placenta. The umbilical cord contains two arteries and a vein, which carry oxygen and nutrients to the fetus and waste products away from the fetus.

  • uterine wall - the wall of the uterus

  • uterus (Also called the womb.) - the uterus is a hollow, pear-shaped organ located in a woman's lower abdomen, between the bladder and the rectum, that sheds its lining each month during menstruation and in which a fertilized egg (ovum) becomes implanted and the fetus develops

  • vagina - the part of the female genitals, behind the bladder and in front of the rectum, that forms a canal extending from the uterus to the vulva

Reasons for the Procedure

An amniocentesis may be used for genetic and chromosome testing in the second trimester of pregnancy in the presence of one or more of these conditions: