At some point, your provider may order a nonstress test (a.k.a. NST) to check on your growing baby. Despite this test’s name, screenings during pregnancy can be stressful. What follows is the full rundown of the NST, in hopes of taking some of the stress out of the process.
What Is a Nonstress Test?
Dr. Carla Williams 1 (MD, FACOG) describes the NST as a method of assessing the baby’s wellbeing. “The reason why we generally want to assess for fetal wellbeing is because there may be a condition in the pregnancy where stillbirth is an increased risk for some reason,” she says. “We’re always trying to see if there is a risk for stillbirth. When there’s a ‘high-risk pregnancy,’ whether that be for fetal conditions or maternal conditions—some examples are multiple gestation, chronic hypertension or gestational diabetes, things like that—we want to do antepartum fetal surveillance.”
It’s called a nonstress test because it doesn’t cause any stress to your baby—plus, it’s noninvasive to you, so there’s not much to worry about.
For the NST, you’ll have to wear two belts with sensors on them around your belly. One sensor picks up your baby’s heart rate, which will be recorded by a machine. The other will sense your contractions. This may be done at your healthcare provider’s office or the hospital.
The NST takes as little as 20 minutes and up to an hour or more, depending on how responsive your baby is.
When is an NST Performed?
Dr. Williams says the timing can vary depending on the provider and institution and based on the risks to the particular person and pregnancy. “If the person has hypertension then that could be around 32 weeks, for example,” she says. “Generally, [NST testing] would start as early as 28 weeks—usually around the time when a pregnancy is viable, the beginning of your third trimester (or later, depending on your condition).”
It could happen even later, too. “For a normal, low-risk pregnancy, providers typically will suggest you have an NST performed around 40 or 41 weeks to assess fetal wellbeing because typically by 41 weeks they’ll be offering induction to limit that risk of stillbirth that occurs later in pregnancy after 41/42 weeks.”
An NST may be ordered at any time if there’s reason to believe your baby is at higher risk for not having enough oxygen. Other factors that may lead to an NST include:
Health conditions: If you have type 1 diabetes, heart disease or high blood pressure during pregnancy, doctors may want to keep a closer eye on your baby.
Past complications: If you’ve had complications in a previous pregnancy, a nonstress test can give you extra peace of mind that your baby is okay.
Rh incompatibility: If your blood is Rh negative and your baby’s is Rh positive and this isn’t your first pregnancy, the doctors may want to monitor the baby using a nonstress test.
Low amniotic fluid: If amniotic fluid is low (known as oligohydramnios), a nonstress test can be used to check on the baby.
Decreased movement: If a baby hasn’t been moving around as often as usual, a nonstress test can help gauge if there’s cause for concern.
Pregnancy with multiples: If you’re pregnant with twins or more and have one or more pregnancy complications, you may get an NST.
Overdue: Past your due date? A nonstress test may be ordered to check that baby’s still doing well while you wait for their much-anticipated arrival.
Nonstress Test Results
There are two results you can get from an NST:
Reactive nonstress test (NST): If during 20 minutes, your baby’s heart rate accelerates a certain amount two or more times, they’re considered “reactive,” meaning there’s no cause for concern. If they don’t have two of those accelerations in 20 minutes, don’t freak out. They might just be sleeping. You’ll have to stick around for at least 20-40 minutes or longer though. And you may get extra stimulation—such as sound waves projected over your belly—to try to wake your baby and get a reaction. “The fetal heart rate has a baseline,” Dr. Williams says. “Let’s say the baseline is 140: we’re looking for a jump of the heartbeat from that baseline. This will depend on gestational age, but before 32 weeks you want a jump of 10 by 10, after 32 weeks you want a jump of 15 by 15, and you want to see that over a period of 20 minutes, at least twice.”
Non-reactive nonstress test (NST): If your baby’s non-reactive, or their heart rate didn’t speed up as desired two or more times in any of the 20-minute windows, it could be because they were sleeping or because of medications you’re taking. In some cases, problems with the placenta or umbilical cord could be causing low oxygen levels, which could make the baby non-reactive.
What happens if you get a non-reactive NST result? If you’re full-term, the doctor might recommend delivering your baby right away. If you’re not full-term, follow-up tests will probably be ordered to figure out why baby’s not reacting. If the nonstress test is read in context of the biophysical profile, other factors will be taken into account as the doctors evaluate your baby’s health.