Ultrasound is a painless way to show a fetus in the uterus. The test uses sound waves and a type of sonar detection system to generate a black-and-white moving picture on a TV screen.
Fetal ultrasound is useful for determining your pregnancy due date and evaluating the health of the baby and its position. It can show some but not all birth defects (for example, it can show some cases of spina bifida. Although some findings on ultrasound are used in screening tests for Down syndrome, it cannot by itself diagnose Down syndrome). It is also useful for diagnosing twins and can sometimes show the sex of your baby.