Amniocentesis is a test to study the liquid (amniotic fluid) that surrounds a baby. Amniotic fluid contains cells and other substances that can give clues about a baby's health. Amniocentesis can be done after about the 15th week of pregnancy, when there is enough amniotic fluid for testing.

Amniocentesis is done by putting a needle through the belly into the uterus. About 30 mL (2 Tbsp) of the amniotic fluid is collected and examined in a lab. The fluid sample can be examined for:

• Genetic characteristics, such as the sex of the baby.
• Chromosome problems, such as Down syndrome and some other birth defects.
• Chemicals that show how mature the baby's lungs are.
• Infection.

Levels of certain substances in the fluid sample, such as alpha-fetoprotein, can also be checked. Some abnormal levels may mean there is a problem with the development of the brain and nerves.

Amniocentesis is usually done between weeks 15 and 20 of the pregnancy for genetic testing. Less often, it is done later in pregnancy to find certain types of infections or other rare problems.