A diagnostic test to determine whether the fetus has any abnormalities. Performed anywhere between weeks 14 and 20 of pregnancy (though more commonly between weeks 16 and 18), amniocentesis tests the fluid inside the amniotic sac that surrounds the baby. The fluid, which contains the baby's skin cells, is drawn from the uterus through a hollow needle inserted through the expectant mother's abdominal wall and is tested for chromosomal abnormalities, genetic abnormalities, or other diseases.