Throughout pregnancy, your baby is surrounded and protected by a fluid-filled sac called the amniotic sac. The amniotic fluid contains cells and many chemicals produced by the growing baby, which provide vital information about the health of your baby before birth. When birth defects in the developing baby are suspected, a sample of this fluid can be removed for analysis by a process called amniocentesis. The procedure involves the insertion of a long thin needle into the amniotic sac under ultrasound guidance to carefully obtain a sample of fluid while avoiding injury to the baby.