Many parents-to-be look forward to an ultrasound examination for their first thrilling glimpse of their baby. 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 pregnant woman’s belly or inserts a probe into her vagina. The woman feels pressure as the provider moves the transducer, but usually no pain.

Ultrasound can be performed at various times during pregnancy:

First Trimester

• 8-12 weeks: to estimate the number of weeks of pregnancy
• 10-13 weeks: to check for a neural tube defect, a birth defect of the brain and spinal cord

Second Trimester

• 16-20 weeks: to help when amniocentesis is being performed
• 18-20 weeks: to check the baby's growth and size and to look for major birth defects

Third Trimester

• To check the amniotic fluid (is there too much or too little?)
• To check the baby's well-being and movement

A routine ultrasound exam, which takes about 20 minutes, can determine the age of the fetus, identify a multiple pregnancy (twins or more), check fetal growth and size, and look for major birth defects. If birth defects are suspected, your provider will refer you for a more detailed ultrasound examination (a level II exam or a 3-D ultrasound).

Ultrasound also is used to help diagnose certain pregnancy complications, such as the causes of vaginal bleeding, and to check fetal well-being late in pregnancy.

When there is a medical reason for the test, ultrasound can provide valuable information leading to treatment that can improve a woman’s chances of having a healthy baby.

Ultrasound is safe for you and your baby. Many providers offer an ultrasound to all low-risk women just to check on fetal growth and well-being. But don’t be too disappointed if your provider doesn’t. Studies suggest that low-risk women who have routine ultrasound exams are no more likely to have healthy babies than low-risk women who don’t have the test.

Ultrasound does not detect all birth defects, so it cannot guarantee a healthy baby. And, occasionally, a woman receives a “false positive result”; that is, she is told incorrectly that her baby may have a birth defect. Although follow-up exams usually show the baby is healthy, false alarms are frightening.