Amniocentesis is a prenatal test that involves taking a sample of amniotic fluid from around the baby in the uterus for testing or treatment. During the procedure, a thin needle is inserted through the abdominal wall and guided by an ultrasound image to remove a small amount of amniotic fluid from the sac surrounding the fetus. The fluid contains fetal cells and chemicals produced by the baby, which can provide useful information about the babys health. Amniocentesis can diagnose certain chromosomal conditions, such as Down syndrome, or genetic conditions, such as cystic fibrosis. It can also be used to detect physical abnormalities, such as spina bifida. The test is usually performed between the 15th and 20th weeks of pregnancy, but it can be done later if necessary. However, having an amniocentesis earlier in pregnancy poses more risks, such as miscarriage. The procedure is not offered to all pregnant women, but only to those who are thought to be at higher risk of delivering a child with a birth defect. Reasons to consider amniocentesis include having positive results from a prenatal screening test, having an earlier pregnancy affected by a genetic condition, or having a family history or previous child with a genetic disease or metabolic disorder. The test is a diagnostic test, which means that it tells for sure whether or not the baby has a health condition. However, there is a small risk of having a miscarriage after the procedure.