Amniocentesis is a test to analyze the amniotic fluid surrounding a fetus. It is done to assess the fetus's health and its risk for certain birth defects. The test is done by gently inserting a needle through the pregnant woman's abdomen into her uterus and withdrawing a small amount of amniotic fluid.

To safely guide the needle during amniocentesis, fetal ultrasound (also called fetal sonography) is used to locate the position of the placenta, the umbilical cord, and the fetus. This helps reduce the risk of injury to the fetus and of puncturing the placenta or umbilical cord.

Ultrasound uses reflected sound waves to produce an image of organs and other structures in the body. For this procedure, gel or oil is applied to the skin to help transmit the sound waves. A small handheld instrument called a transducer is passed back and forth over the abdomen. The transducer sends out high-pitched sound waves (above the range of human hearing) that are reflected back to the transducer. A computer analyzes the sound waves and converts them into a picture of the fetus and surrounding area.