What is a non-stress test?

Q: I'm a week past my due date, and my doctor wants to do a non-stress test. Can you explain what this is?

A: The non-stress test is an opportunity to "interview" the fetus to ensure it is doing well. Typically the test is performed in the third trimester to assess various situations, such as if a woman has passed her due date or has certain medical conditions, including high blood pressure and diabetes. We also might employ this test when there are concerns about the fetus's well-being, such as a perceived decrease in movement.

The test involves placing a belt with two monitors around the woman's abdomen. One monitor records the fetal heart rate; the other records uterine contractions. Once we've established the baby's baseline heart rate, we evaluate it for 20 minutes to an hour to accommodate the baby's sleep-wake cycle; we look for the rate to increase by 15 beats per minute for at least 15 seconds. We want this acceleration to occur twice over the course of the test. The test is then classified as either reactive—meaning we saw an acceleration—or nonreactive. A nonreactive stress test is not necessarily cause for concern, but your doctor may want to follow up with additional testing.