A nurse is providing teaching to a client who is at 32 weeks of gestation about a nonstress test

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A nonstress test (NST) is a test during pregnancy that measures your baby’s heart rate and response to movement. Designed to make sure she’s doing well and getting enough oxygen, your provider might order it during your third trimester if you’re experiencing certain complications or have zoomed past your due date.

This prenatal test is safe, straightforward and noninvasive — the goal is simply to see how your baby responds when no stress is placed on her (hence the name!).

Of course, needing a nonstress test might make you stressed. But knowing what to expect and how to make sense of the results may give you some peace of mind.

Here’s why nonstress tests are done, who might need one, how they work and what the findings mean for you and your baby. 

Who needs a nonstress test during pregnancy?

A nonstress test is done in the third trimester of pregnancy, from week 28 on. There are a number of reasons why your provider might order one — and they don’t automatically mean something is wrong. 

You might have an NST if:

• Your due date passes but your baby stays put.
• Your baby is measuring small for her gestational age or isn't very active.
• You have a complication like gestational diabetes or preeclampsia, or an underlying medical condition like type 1 diabetes, heart disease or high blood pressure.
• You’re having multiples and have certain related complications.
• You had complications during a previous pregnancy.
• You and your baby have Rh incompatibility.
• Your amniotic fluid levels are low.
  

How to prepare for a nonstress test

You don’t have to do anything special before a nonstress test. But your provider might recommend having a snack beforehand, since your baby is likely to be at her wriggliest shortly after you’ve eaten. And since the test can last up to 40 minutes, it’s worth making a trip to the bathroom before heading in for your exam.  

As for what to wear? Comfortable, loose-fitting clothing that allows for easy access to your belly is best, so your provider can place the sensor around your abdomen without too much trouble. You might also be asked to change into a gown.

If you take any medications, check with your practitioner about whether they’re okay to take before your nonstress test. Since some drugs can make babies less reactive, depending on your prescription, your doctor might recommend skipping a dose.

What happens during a nonstress test

Nonstress tests typically take 20 minutes, but they can last as long as 40 minutes if your baby isn’t very active or seems to be asleep. In that case, your provider might try to stimulate your baby partway through the test by having you drink something sugary, eat a snack or placing a noise-making device near your stomach.

During the test, a stretchy belt monitor will be placed around your belly (the same kind doctors often use during labor and delivery). It shouldn't hurt and doesn't usually feel uncomfortable.

You’ll lie back on a reclining chair, but there won’t be any mindless magazine-flipping. You've got a job to do: You'll be asked to keep track of each movement you feel baby make.

You may hold a clicker contraption (like a buzzer on Jeopardy), and each time you feel the baby move, you'll click it. Other fetal monitors may work differently.

What nonstress test results mean

Your baby’s performance on the nonstress test will be classified as reactive or nonreactive. And in most cases, your provider can tell you about the results and what they mean as soon as the test is over. 

Reactive

A baby who moves a lot and has a normal heart rate is classified as "reactive" — i.e. healthy and not under any stress.

If your test happens before 32 weeks, “reactive” means your baby’s heart rate went above her baseline twice for at least 10 seconds during the exam. The time increases to 15 seconds if your test happens after 32 weeks.

Nonreactive

A "nonreactive" baby is one who doesn't make a minimum number of movements during the 40-minute period or whose heart doesn't accelerate as much as expected when she does move.

A nonreactive NST result can make you anxious — but it doesn’t necessarily mean there's something wrong. Sometimes babies are just sleepy or less active. In some cases, however, it could mean your baby isn't getting enough oxygen.

With a nonreactive result, your provider might opt to do the test for another 40 minutes while encouraging your baby to move more, either with sound, a snack or a sugary drink.

If your practitioner concludes that your baby could be under stress, you might need to undergo further testing (like a biophysical profile) to determine whether labor induction and early delivery are necessary.

Nonstress test risks

Good news: Nonstress tests are noninvasive and completely safe. They don’t come with any physical risks for you or your baby.

That said, undergoing the test can certainly make you worried or stressed. And research suggests that in some cases, extreme anxiety can potentially affect your results.

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