A nonstress test (NST), also known as fetal heart rate monitoring, is a common prenatal test used to check on a baby's health. During a nonstress test a baby's heart rate is monitored to see how it responds to the baby's movements. The term nonstress refers to the fact that nothing is done to place stress on the fetus during the test.
Typically, a nonstress test is recommended when it's believed that the fetus is at increased risk of death. A nonstress test may be done after 26 to 28 weeks of pregnancy. Certain nonstress test results might indicate that you and your baby need further monitoring, testing or special care.
A nonstress test is a noninvasive test that doesn't pose any physical risks to you or your baby. Find out what a nonstress test involves and whether this prenatal test might benefit you or your baby.
Why it's done
A nonstress test is used to evaluate a baby's health before birth. The goal of a nonstress test is to provide useful information about your baby's oxygen supply by checking his or her heart rate and how it responds to your baby's movement. The test might indicate the need for further monitoring, testing, treatment or delivery to prevent fetal death.
Normally, a baby's heart beats faster when he or she is active later in pregnancy. However, conditions such as fetal hypoxia — when the baby doesn't get enough oxygen — can disrupt this pathway.
Your health care provider might recommend a nonstress test if you have:
- A multiple pregnancy with certain complications
- An underlying medical condition, such as type 1 diabetes, heart disease or high blood pressure during pregnancy
- A pregnancy that has extended two weeks past your due date (postterm pregnancy)
- A history of complications in a previous pregnancy
- A baby who has decreased fetal movements or possible fetal growth problems
- Rh (rhesus) sensitization — a potentially serious condition that can occur, typically during a second or subsequent pregnancy, when your red cell antigen blood group is Rh negative and your baby's blood group is Rh positive.
- Low amniotic fluid (oligohydramnios)
Your health care provider might recommend having nonstress tests once or twice a week — and occasionally daily — depending on your and your baby's health. For example, you might need to have regular nonstress tests if your health care provider suspects that your baby is at risk of not getting enough oxygen. You might also need another nonstress test if you or your baby has any concerning changes in health.
A nonstress test is a noninvasive test that poses no physical risks for you or your baby. The term nonstress refers to the fact that nothing is done to place stress on the fetus during the test.
While a nonstress test can offer reassurance about your baby's health, it can cause anxiety, too. A nonstress test also might not detect an existing problem or might suggest that a problem exists when there is none, which can lead to further testing.
Also, keep in mind that while a nonstress test is often recommended for women who have an increased risk of pregnancy loss, it's not always clear if the test is helpful.
How you prepare
A nonstress test typically requires no special preparation.
What you can expect
A nonstress test is usually done in your health care provider's office.
Before the procedure
You'll have your blood pressure taken before the nonstress test begins.
During the procedure
During the nonstress test, you'll lie on a reclining chair. You'll have your blood pressure taken at regular intervals during the test.
Your health care provider or a member of your health care team will place two belts with monitors attached to them across your abdomen. One belt will record your baby's heart rate, and the other will record any uterine contractions you might have. You'll be asked to note when your baby moves. Your baby's movements will be noted on the fetal heart record. Your health care provider will look to see if your baby's heart beats faster when he or she moves.
Typically, a nonstress test lasts 20 minutes. However, if your baby is inactive or asleep, you might need to extend the test for another 20 minutes — with the expectation that your baby will become active — to ensure accurate results. Your health care provider might try to stimulate the baby manually or by placing a device on the maternal abdomen that makes a noise.
After the procedure
After the nonstress test is complete, your health care provider will likely discuss the results with you right away.
Results of a nonstress test are considered:
- Reactive. Before week 32 of pregnancy, results are considered normal (reactive) if your baby's heartbeat accelerates to a certain level above the baseline twice or more for at least 10 seconds each within a 20-minute window. At week 32 of pregnancy or later, if your baby's heartbeat accelerates to a certain level above the baseline twice or more for at least 15 seconds each within a 20-minute window, the results are considered reactive.
- Nonreactive. If your baby's heartbeat doesn't meet the criteria described above, the results are considered nonreactive. Nonreactive results might occur because your baby was inactive or asleep during the test.
If the test is extended to 40 minutes and your baby's nonstress test results are nonreactive and you are 39 weeks pregnant (full term), your health care provider might recommend delivery. If you are not full term, your health care provider will likely do another prenatal test to further check your baby's health. For example:
- Biophysical profile. A biophysical profile combines a nonstress test with a fetal ultrasound that evaluates your baby's breathing, body movements, muscle tone and amniotic fluid level.
- Contraction stress test. This test looks at how your baby's heart rate reacts when your uterus contracts. During a contraction stress test, if adequate uterine activity doesn't occur on its own you will be given intravenous oxytocin or be asked to rub your nipples to induce uterine activity.
Your health care provider might also ask you to have another nonstress test later in the day. Keep in mind that a reactive result is far more likely to be correct than is a nonreactive result. If you have a nonreactive nonstress test and a second nonstress test that's reactive, the results of the second test are considered reliable.
Other possible causes of a nonreactive nonstress test result, besides the fetus being inactive or asleep, include decreased oxygen (fetal hypoxia), maternal smoking, maternal use of medications, and fetal neurologic or cardiac anomalies.
Rarely, during a nonstress test, problems with a baby's heart rate are detected that require further monitoring or treatment.
Be sure to discuss the results of your nonstress test with your health care provider and what they might mean for you and your baby.
Feb. 23, 2016