What is a nonstress test?
A nonstress test is a simple, noninvasive procedure that involves monitoring your baby's heartbeat to make sure your baby is getting the oxygen he needs through the placenta.
This test helps your doctor determine if your baby is distressed and make plans for delivery if he is.
When is a nonstress test performed?
The test is usually recommended in the third trimester if your pregnancy is considered high risk or if you have a low-risk pregnancy but are already more than a week past your due date. In either situation, your healthcare provider may want to perform the test once or twice a week, or more often if necessary. The nonstress test can be performed as early as 24 weeks, but generally is not performed until the 28th week as the baby is not developed enough to respond to the testing method.
If you have diabetes, high blood pressure, kidney disease, or heart problems, or if you're carrying twins or multiples, your doctor may suggest you have this test.
Your doctor may also recommend this test if an ultrasound picks up that you have too much or too little amniotic fluid around your baby, or if your baby is not moving as much as expected.
What happens during the test?
During the nonstress test, two belts are placed around your stomach. One belt records your contractions. (Contractions may be a sign of preterm labor if you're less than 37 weeks pregnant.) The other belt is attached to a monitor that records your baby's heart rate and movement. When the baby moves, his heart rate should increase -- just as yours does when you move around or exercise. Every time you feel the baby move, you push a button.
What do the results mean?
If your baby is moving and active, his heart beats faster by at least 15 beats per minute. This happens for at least 15 seconds at a time. In order to be considered normal or "reactive," this activity has to happen at least twice in 20 minutes. If your baby isn't moving much, the test is considered "nonreactive" or abnormal.
If the baby doesn't move during the 20- to 40-minute test, it's no cause for worry. He or she may just be asleep. Your doctor or a medical technician may try to wake the baby with a buzzer or by having you eat or drink to stimulate movement. Your doctor may want also to repeat the test every week -- or more often -- until your baby is born, just to play it safe.
This test sometimes delivers false positive results if it is performed early in the third trimester. As many as 50 percent of babies tested during their 24th to 28th weeks may have an abnormal result on a nonstress test, according to one study. And another 15 percent of tests conducted on babies in the 28th to 32nd week gestation may also produce an abnormal test result.
Talk to your doctor about what these results really mean and other tests that can rule out problems. Your doctor may repeat the test or perform additional ultrasounds to measure your baby's heart rate.
If the test is nonreactive, that doesn't mean something is wrong. It just means that additional tests might be needed to determine whether or not there is anything to worry about. If the test is reactive (normal), it's very reassuring and unlikely that the baby is in any immediate danger.
Johns Hopkins Medicine. Understanding the Non-Stress Test. http://womenshealth.jhmi.edu/ob-ultrasound/antepartum_testing/antepartum_home.html
American College of Obstetricians and Gynecologists. Practice Bulletin, Clinical Management Guidelines for Obstetricians-Gynecologists, No. 9.
Harvard University Health Services. Prenatal Screening Tests. http://huhs.harvard.edu
March of Dimes. Amniotic Fluid Abnormalities. http://www.marchofdimes.com/professionals/681_4536.aspm
American College of Obstetricians and Gynecologists. Planning Your Pregnancy and Birth. Third Edition, 2000 p. 406-408
American Family Physician. Preterm Labor. Vol. 59/No. 3. http://www.aafp.org/afp/990201ap/593.html
American Pregnancy Association. Fetal Non-Stress Test (NST). http://www.americanpregnancy.org/prenataltesting/non-stresstest.html