What is the test?
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.
How do I prepare for the test?
Early in the pregnancy, you may be asked to drink several glasses of water an hour before the ultrasound so that the uterus can be seen more clearly (a full bladder moves the intestines out of the way). Later in the pregnancy, the uterus has grown sufficiently in size so that this is not necessary.