Amniocentesis

What Can Amniocentesis Reveal?

There are two types of tests that can be performed on amniotic fluid to show different types of problems with the fetus:

• Genetic amniocentesis
• Chemical tests

The cells in the amniotic fluid contain genetic material that can be used to observe chromosomal abnormalities or defects in specific genes. Biochemical substances, like proteins and lipids, can give clues about the health or maturity of the fetus.

Genetic Amniocentesis

The need for genetic testing of amniotic fluid depends on:

• The age of the parents
• Family history of genetic abnormalities
• Problems found during blood tests on the mother
• Ultrasound results

For genetic testing, fetal cells collected by amniocentesis, isolated, and allowed to grow in the laboratory and divide. The chromosomes, which carry genetic material, are stained with dye and examined under a microscope. Normally, every cell has two copies of each of the 23 human chromosomes (46 total).

A commonly detected abnormality is Down syndrome, where a baby has trisomy 21 – an extra copy of chromosome 21. Other trisomy chromosome abnormalities include Edward’s syndrome, where there is an extra copy of the 18th chromosome, and Patau’s syndrome, where there is an extra copy of the 13th chromosome.

Sex chromosome abnormalities can also be detected. Some, likeTurner syndrome – where a girl has a single X chromosome – have significant consequences, such as a high chance of dying before birth, abnormalities in some organs, and infertility. Other sex chromosome abnormalities, such as the presence of an extra X chromosome in either sex or of an extra Y chromosome in males, have relatively slight consequences.

The chromosome test can also reveal the sex of the fetus, since males have an X and a Y chromosome, while females have two X chromosomes.

Chemical Tests

Many other tests can also be performed on amniotic fluid. A test for the presence of alpha-fetoprotein (or AFP) reveals whether there is an anomaly of the abdominal wall or the spine of the fetus, with aneural tube defect, such as spina bifida or anencephaly.

Other diseases, such as Tay-Sachs disease and sickle-cell disease, are detected by the presence of a specific chemical compound in the amniotic fluid. Many other gene abnormalities of the fetus can also be tested in amniotic fluid.

When a mother is Rh negative, meaning her red blood cells lack a specific protein on their surface, amniocentesis can reveal if the fetus is Rh positive. The resulting fetal anemia from destruction of red blood cells in Rh immunization disease can also be detected from levels of bilirubin in amniotic fluid.

In the second and third trimesters, analyzing the amniotic fluid can help show how mature the fetal lungs are by measuring the fetal lung lipid (surfactant) in the fluid. This can help prevent a premature delivery. In a pregnancy complicated by diabetes, monitoring the levels of glucose and insulin in amniotic fluid can be helpful. In cases of preterm labor or preterm ruptured membranes, checking for the presence of bacteria in the amniotic fluid is an important diagnostic test.

For more detailed information about preterm labor, go to Premature Labor.