Who: Most pregnant women.

Why: To check the age of the baby, identify a multiple pregnancy (twins or more), check the baby’s growth and size, and look for major birth defects. Also used to help diagnose certain pregnancy complications.

When: Usually at 18-20 weeks; at other times, as needed.

How: Ultrasound uses sound waves to show a picture of the baby on a screen. The health care provider rubs a handheld device, called a transducer, across the woman’s belly or inserts a probe into her vagina. The woman feels pressure as the provider moves the transducer, but usually no pain.

Risks: Considered safe for the mother and baby when properly used and when medical information about pregnancy is needed. Non-medical use of ultrasound during pregnancy should be avoided.

For more information, read the March of Dimes fact sheet on ultrasound.