NST - Non-Stress-Test during pregnancy
A non-stress test, or NST, is a test that measures your baby’s heart rate and response to movement. It is designed to make sure your baby is doing well and getting enough oxygen. Your doctor may order it in the third trimester if you have certain complications or are well past your due date.
The nonstress test is safe, simple and non-invasive – the goal is simply to see how your baby responds when not under stress. (Hence the name!)
Of course, needing an NST can stress you out. But if you know what to expect and how to interpret the results, you can rest easy. Here’s why non-stress tests are done, who might need one, how they work, and what the results mean for you and your baby.
Who needs a nonstress test during pregnancy?
An NST is performed in the third trimester of pregnancy, starting at 28 weeks. There are a number of reasons your doctor might order an NST – and they don’t automatically mean something is wrong.
An NST may be performed if:
- Your due date passes, but your baby remains there
- Your baby is too small for his or her gestational age or is not very active
- You have a complication like gestational diabetes or preeclampsia, or an underlying condition like type 1 diabetes, heart disease or high blood pressure
- You are having multiples and have certain complications
- You had complications during a previous pregnancy
- You and your baby have rhesus incompatibility
- Your amniotic fluid levels are low
How to prepare for a nonstress test
You don’t have to do anything out of the ordinary before a non-stress test. But your doctor may suggest you have a snack beforehand, as your baby is likely to be most fidgety shortly after eating. And since the test can take up to 40 minutes, it’s worth making a quick trip to the bathroom before you go in for the exam.
Nonstress test risks
Good news: Stress tests are non-invasive and perfectly safe. They pose no physical risks to you or your baby.
Still, taking such a nonstress test can naturally make you anxious or stressed. Research suggests that in some cases, extreme anxiety can affect the results.
If you are very stressed about the test and feel that you are having difficulty coping, talk to your physician. Together, you can speak about what’s on your mind and find ways to deal with your anxiety.
Fetal heart rate within the normal range
During the 9th or 10th week, your baby’s heart rate is around 170 beats per minute, which will gradually decrease for the remainder of your pregnancy. The heart rate will drop to 140 beats per minute by week 20.
There can be transient changes outside of the usual fetal heart rate range during labor for a variety of reasons (often entirely acceptable).
What happens during a nonstress test
Non-stress tests usually last 20 minutes. However, they can take up to 40 minutes if your baby is not very active or seems to be sleeping. In this case, your doctor may try to stimulate your baby during the test by having you drink something sugary or by placing a noise-making device near your belly.
During the test, a stretchy belt will be placed around your belly (the same kind doctors often use during labor and delivery). It should not hurt and usually does not feel uncomfortable. Your health care provider or a member of your health care team will place a sensor around your abdomen that measures the fetal heart rate.
You will recline on a recliner, but there will be no mindless flipping through magazines. You have a job to do: you will be asked to record every movement the baby makes. You can hold a clicking device (like a buzzer on Jeopardy) in your hand; every time you feel the baby move, you will click it. However, other fetal monitors may work differently.
What the results of the non-stress test NST mean
Your baby’s performance on the non-stress test will be classified as reactive or non-reactive. And in most cases, your doctor can tell you the results and what they mean as soon as the test is finished.
A baby who moves a lot and has a normal heart rate is classified as “reactive” – meaning he or she is healthy and not under stress. If your test is before 32 weeks, “reactive” means your baby’s heart rate was above baseline for at least 10 seconds twice during the test. This time period increases to 15 seconds if your test takes place after 32 weeks.
A “non-reactive” baby is one who does not make the minimum number of movements during the 40-minute period or whose heart does not accelerate as much as expected when he or she moves.
A non-reactive result may worry you – but it doesn’t necessarily mean your baby is in danger. Sometimes babies are just sleepy or less active. In some cases, though, it may mean your baby isn’t getting enough oxygen.
In the case of a non-reactive result, your doctor may decide to run the test for another 40 minutes while encouraging your baby to move more, either with sounds or a sugary drink. If it is determined that your baby may be under stress, you may need to undergo further testing (such as a biophysical profile) to determine if labor induction and early delivery are necessary.
A reactive non-stress test is considered reassuring regardless of the length of observation time. However, if the test is prolonged to 40 minutes and your baby’s non-stress test results are not reactive, your doctor will likely perform another prenatal test to further check your baby’s health. For example:
Contraction stress test.
This test looks at how your baby’s heart rate responds when your uterus is contracting. If sufficient uterine activity does not occur on its own during a contraction stress test, you will be given intravenous oxytocin or asked to rub your nipples to induce uterine activity.