Ultrasound scan to amniocentesis test. Amniocentesis is a diagnostic procedure performed by inserting a needle (seen on the left) through the abdominal wall into the uterus and withdrawing a small amount of fluid from the sac surrounding the fetus. The test can detect chromosomal disorders, such as Down syndrome, structural defects, such as spina bifida (open spine, where the vertebrae fail to close), anencephaly (a condition in which the brain is incomplete or missing), and many rare, inherited metabolic disorders. © Mediscan/Corbis