What is a Non-Stress Test (NST) in Pregnancy?

Pregnant Mom with monitors on her belly, non-stress test

There are a variety of tests that your doctor may recommend at some point during pregnancy. After 28 weeks of gestation, one of them may be the non stress test – a pregnancy screening that measures fetal heart rate and reaction to movement. It is referred to as a non-stress test because no stress is placed upon the baby during the test. 

Your baby’s heart rate can fluctuate quite a bit when they are in the womb –  just like when you run, your heart rate increases. So too, your baby’s heart rate increases when they kick or move around. During the test, they will encourage your baby to move. While your baby moves, they will track their heart rate to analyze the information. 

When is it needed?

While your baby develops, you attend your prenatal appointments, listen to your baby’s heartbeat, and you may see your baby on an ultrasound. There are signs and risk factors that your doctor or midwife knows to look out for. 

They may suggest a non-stress test if they observe anything out of the ordinary, so they can monitor your baby further. 

Here are a few reasons why an NST may be performed? 

1. If you sense your baby isn’t moving as frequently as usual. 

You will start to feel your baby moving during the second trimester (between 16 to 24 weeks). By 32 weeks, it’s normal to feel your baby moving at least every day. Every baby is unique and you will get to know your baby’s individual movement patterns. 

You may even start to track your baby’s movement. Your baby will continue to move daily right up until the day they are born. Feeling regular movements is a sign that your baby is well. 

If you notice that your baby isn’t moving as much as they were before, you might want to bring this up to your doctor or midwife.  

Remember: You know your baby best.

Once you learn your baby’s pattern– you are the one most in tune with their movement patterns. If you feel something is off, tell your doctor or midwife. 

They’ll likely recommend a non-stress test to get more information. 

2. You are overdue (past 40 weeks of pregnancy.) 

Phew…going past 40 weeks of pregnancy can be mentally challenging, but it’s quite normal. Your care provider may recommend a non-stress test to check on your baby and make sure they’re still doing great. If they have any concerns they will recommend further action, or send you on your way to continue patiently waiting for the big day! Hang in there, you will meet your baby soon. 

3. If there is any reason to suspect your placenta is not functioning adequately. 

Your doctor might recommend an NST if they suspect complications with your placenta. Your placenta passes oxygen, nutrients and antibodies from your blood to your baby. It also carries waste products from your baby back to your blood, so your body can get rid of them. The placenta’s job is vital to your baby’s growth and development. 

4. Rh Sensitization. 

When you have blood work done during your first trimester of pregnancy, you are often tested for the Rh factor. This is a protein found on the surface of red blood cells. If you have this protein, you are Rh-positive. If you don’t, you are Rh-negative

Now, being Rh-negative isn’t an issue by itself. But during pregnancy, if you are Rh negative, and your baby is Rh positive, it may cause issues or Rh sensitization. While this is rare, Rh sensitization happens when your blood develops antibodies against your baby’s blood. 

This can cause a variety of issues for your baby. So,if you happen to be Rh negative, your doctor may order a non-stress test to check on and make sure your baby is healthy.  

5. You are high-risk for any other reason. 

A high-risk pregnancy is a pregnancy with a “higher-than-normal” chance of having complications. If you have any high-risk conditions you may receive non-stress tests regularly towards the end of your pregnancy to see how your baby is doing.  

Some potential maternal high risk factors include: 

  • Pre-existing diabetes
  • Organ transplant 
  • High blood pressure
  • Lupus
  • Cancer
  • Blood clots
  • Heart disease 
  • HIV or hepatitis

Also, potential fetal risk factors

  • Inherited diseases 
  • Fetal anemia (not enough red blood cells to carry oxygen)
  • Chromosome issues 
  • Small for gestational age

And lastly, risk factors associated with pregnancy

How is this test done? 

It is non-invasive and does not have any known side effects or risks to you or your baby. 

While you are lying down…

  • Your doctor will use a machine called a fetal monitor 
  • They will attach one elastic belt around your abdomen to measure your baby’s heart rate, and another belt to measure any potential contractions you may be having. 
  • Your baby’s movement, heart rate, and reactivity will be measured for about 20 to 30 minutes.
  • If your baby isn’t moving during the test, they may use a small buzzer near your belly to wake the baby.

After the test…

Your doctor will explain the results to you. This will also be your time to clarify questions and discuss concerns. 

What is the test going to detect? 

Here are the only two test results you can receive: 

“Reactive NST” or “Non-Reactive NST”. 

A reactive result means your baby’s heart rate increased to at least 15 beats per minute over the baseline (120 to 160 beats per minute). This must last at least 15 seconds within a 20-minute period. 

A non-reactive result means your baby’s heart rate did not rise as much during the test. 

Yes, reactiveness is what we are looking for. 

But, if your results come back as “non-reactive” it doesn’t necessarily mean anything is wrong. It simply warrants the need for more tests. This way, any issues can be confirmed or ruled out. 

Remember, the main goal of this test is to measure the heart rate of your baby in response to their own movements. 

A normal heart rate will increase during times of movement and decrease while resting. So, if your baby is “non-reactive” it may indicate an issue with their oxygen levels. 

So, a “non-reactive” test result may mean nothing. But, it may call for additional testing, which your doctor will discuss with you.

Feeling stressed about your baby’s health? 

Reach out to your doula for some extra guidance.We are here for you. No question is too “out there.” Your doula’s job is to help you get the information you need, reduce fear and instill confidence and peace. You can contact us anytime, here.

About Christie Collbran

Christie believes in helping women recognize their own inner wisdom, strength and power. Having served as President of the Tampa Bay Birth Network for six years and with ten years serving families as a birth doula, she has a reputation for leadership, dedication and compassion. A childbirth educator, certified lactation counselor as well as a certified doula, she makes a point of ensuring mothers and their partners understand all their birthing options and what to expect on their journey.> keep reading