Originally published by Harvard Health.
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.
What happens when the test is performed?
You lie on your back on a table for the test. A technician or doctor first squirts some clear jelly onto your lower abdomen to help the ultrasound sensor slide around easily, and then places the sensor against your skin. The sensor looks like a microphone. When the sensor is in place, a picture appears on a TV screen, and the technician or doctor moves the sensor back and forth on your abdomen to see the fetus from many different views. Depending on the age of the fetus, the heartbeat, movement of arms and legs, and many of the internal organs may be seen.
What risks are there from the test?
There are no risks to either the mother or the fetus from ultrasound.
Must I do anything special after the test is over?
The jelly is wiped off. There are no side effects from the test.
How long is it before the result of the test is known?
Depending on whether the person doing the test is a doctor or a technician, you might be able to get a few results of your test immediately. You may even be given a printed picture of your baby as seen by ultrasound, to take home. The test is recorded on a videotape; a radiologist can review the tape and report to your doctor, a process that might take a day or two.