NST: Non Stress Tests During Pregnancy, Explained!

Did your provider tell you they want you to come in for an NST? Wondering what to expect and why non stress tests happen during pregnancy? You’re in the right place!

NSTs are a fairly common, non-invasive test that happen during pregnancy. Basically, we’re looking to see if baby’s heart rate seems normal, if you’re having any contractions (including Braxton Hicks), and how well baby is tolerating any contractions that ARE happening.

My provider asked me to come in for an NST when I hit my due date with both of my boys (yep, I went a few days past due both times!). She just wanted to get a sense of how baby was doing in there once I hit 40 weeks.

But that’s just one reason you might need an NST! Read on to learn about when and why we use them, and more details about how the whole process works!

What exactly is a non stress test?

In short, non stress tests (aka cardiotocography) use monitors to check on baby’s heart rate and movements, and to see if you are experiencing any contractions.

If contractions ARE present, the NST helps to check how well baby is tolerating those contractions. These can include traditional labor contractions, Braxton Hicks contractions, and the contractions felt during prodromal labor, too.

What is being tested?

During an NST we will be looking for any changes in baby’s heart rate based on whether they are moving, resting, or experiencing a contraction.

Like with all living things, we expect baby’s heart rate to increase when they are moving or responding to a contraction, but we do want to make sure that it’s not causing TOO much of an increase either.

After the test is complete, your results will be described as reactive or nonreactive. Here’s an overview of these outcomes based on description from The Mayo Clinic.

Reactive

A reactive test result means that everything seems normal. Your test results will be classified as reactive in different ways based on where you’re at in your pregnancy:

  • Before week 32: Baby’s heartbeat rose to a certain threshold above baseline two or more times, for at least 10 seconds, within a 20-minute window (often, that’s the duration of the test)
  • After week 32: Baby’s heartbeat rose to a certain threshold above baseline two of more times, for at least 15 seconds, within a 20 minute window

Nonreactive

A nonreactive result basically means that the above criteria wasn’t met. Sometimes, that means baby was inactive or asleep, and that everything is still okay, but other times it can indicate a problem.

Usually, with a nonreactive result, your provider will have you stay on the monitor for a longer period of time, and if results are still nonreactive, a follow-up test will be needed to check on baby’s health and stability (more on these below!).

When is a NST necessary? Reasons for an NST

So, when is an NST necessary? NSTs can be used anytime after week 26-28 of pregnancy, and can happen for many reasons. They are actually one of the more common types of prenatal tests, because they’re non-invasive and don’t stress baby!

Let’s take a look at the common reasons: (source)

You’ve reached your due date

It’s really common for your provider to have you come in for an NST once you’ve reached 40 weeks. They get a sense of how baby is doing in there and whether or not you’re having any light contractions.

It can help assess any need for follow up tests or if an induction could be necessary for baby’s safety.

You notice a decrease in baby’s movement

Once you hit week 28 of pregnancy, it’s really important to tune into your baby’s daily movements. The best way to do this is with kick counting, where you lie quietly for 2 hours and try to feel 10 movements.

But additionally, you want to be aware of your baby’s typical movement patterns and remember that just because baby isn’t getting bigger DOESN’T mean they will move less.

If you have any sense that baby is moving less than usual, definitely call your provider. In fact, this is one of the top 5 reasons I tell mamas to always make a call. We would so much rather you come in and have an NST done that checks out totally normal than have an emergency go unnoticed.

So yeah. Kick counting matters!

You have certain preexisting conditions

Sometimes your provider will recommend regularly scheduled NSTs throughout your third trimester if you have certain preexisiting conditions that could put you or baby at risk for other complications.

Some of these include:

  • Diabetes
  • High blood pressure
  • Heart disease
  • Clotting disorder
  • Other blood disorders
  • Thyroid disorders

You have current pregnancy complications or conditions

And on a similar note, certain conditions or complications that come on during pregnancy can result in more NSTs during your pregnancy! In short, anything that causes your pregnancy to be considered “high-risk” often meets once or twice weekly NSTs to make sure baby is okay in there!

Some of these reasons include:

You had complications in a past pregnancy

When mamas have experienced complications in past pregnancies, that can be another reason for more frequent NSTs in their current pregnancies. This includes things like a history of preterm birth, stillbirth, and recurrent pregnancy loss.

What to expect during an NST

Non Stress Tests are usually performed in-office, but sometimes you’ll go into your hospital’s labor and delivery unit. Most often, the test will last for 20 minutes, and if at that point there is a nonreactive result, they may extend the test to last anywhere from 40-90 minutes before discussing the next steps.

Here’s what you can expect:

  • When you arrive, you’ll be asked to lie back in a reclining chair or on a reclining table (you know – like the ones in the exam rooms), and some providers have you lie on your left side with a wedge
  • You will have two monitors placed onto your belly using a stretchy band. One is to measure baby’s heart rate, and the other is to measure contractions. These are the same types of monitors that are routinely used when you come in to deliver, too
  • During the test, you’ll be in charge of noting any movement you feel. Usually, this is done with a little remote button, but sometimes you’ll chart it on paper
  • If baby is nonreactive in the first 20 minutes, you may be instructed to move around, eat a snack, or drink something cold before resuming, to see if that helps to get baby moving

Any special way to prepare for a non stress test?

My best advice is to dress in something comfortable that allows for easy access to the belly. I opted for maternity leggings and a stretchy cotton shirt. And to have a little snack or a meal before you come in, this is something you might be instructed to do!

You also want to make sure to use the bathroom right before the test starts so you don’t need to pause or wait uncomfortably for things to finish. Some places will even cue you to empty your bladder before things start ?

What about a pregnancy stress test (CST)?

So this is actually something totally different, but it often comes up when I’m talking about NSTs!

Stress tests during pregnancy, also known as an oxytocin challenge test or contraction stress test (CST), are done to see how well baby tolerates the stress of a contraction.

CSTs are far less common than they used to be, because NSTs and other options are often a better choice. Pregnancy stress tests are more commonly used as follow-ups to a nonreactive NST.

How do they work?

This is almost always performed at the hospital because we use an IV with a small amount of Pitocin to stimulate some contractions. The same two monitors are strapped to your belly, and we will look at the way baby’s heart rate reacts during a contraction.

It’s usually only performed when mama has a high-risk pregnancy with complications that could make it more difficult for baby to tolerate the stress of contractions. Or when results of another third-trimester test indicate the need for a contraction stress test.

Other third trimester tests used for follow-up to the NST

So we just talked about stress tests, and how they might be used as a follow-up to an NST, but there are other third trimester tests that might come up too, including:

  • Contraction stress test (CST): This is the one we just talked about above!
  • Biophysical profile (BPP): This test uses an ultrasound to monitor baby’s movement, body tone, breathing efforts, and amniotic fluid levels. These elements are given a score between 0-2. Then the score is added up to determine if baby is in good shape to stay in, needs to be delivered, or if a repeat test is needed
  • Amniotic fluid Index (AFI): In this test, we look at the amount of fluid around baby, you can learn more about it in this article

Ready for your NST?

Now that you understand why they happen and what you can expect from your non stress test, you know that there’s really no reason to stress at all!

This is a simple, non-invasive test that can help your provider quickly assess baby’s health and well-being. Do you have any more questions about your upcoming NST? Feel free to drop a comment below!

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Liesel Teen, BSN-RN
Founder, Mommy Labor Nurse

Meet Liesel Teen

Hi there. I’m Liesel!

As a labor and delivery nurse, I’ve spent countless hours with women who felt anxious — even fearful — about giving birth. I want you to know it doesn’t have to be that way for you!

When you know what to expect and have the tools to navigate the experience, you’ll feel confident and in control.

I believe you deserve a better birth — no matter how you deliver.