The contraction stress test (CST) was used to attempt determination of fetal reserve prior to labor by evaluating the response of the fetal heart rate to spontaneous or induced uterine contractions. Testing was performed by using an external fetal heart rate monitor and tocograph. The CST was employed 189 times in 120 high-risk patients who either had hypertensive disorder of pregnancy or had completed at least 42 weeks of gestation. Testing was begun as early as 34 weeks’ gestation and repeated at weekly intervals; it was not used in the clinical management of the patient. The maximum number of tests performed on one patient was 8. A negative CST appears to be a most reliable guide to the ability of the fetus to tolerate labor if it ensues within 1 week. A positive test suggests that the fetus is at increased risk, but death is not necessarily imminent. Results of the present study warrant use of the CST in controlled studies to determine its effect in improving perinatal outcome.
Overview
A nonstress test is a common prenatal test used to check on a baby’s health. During a nonstress test, the baby’s heart rate is monitored to see how it responds to the baby’s movements. The term “nonstress” refers to the fact that nothing is done to place stress on the baby during the test.
Typically, a nonstress test is recommended when it’s believed that the baby is at an increased risk of death. A nonstress test may be done after 26 to 28 weeks of pregnancy. Certain nonstress test results might indicate that you and your baby need further monitoring, testing or special care.
A nonstress test is a noninvasive test that doesn’t pose any physical risks to you or your baby.
Why it’s done
A nonstress test is used to evaluate a baby’s health before birth. The goal of a nonstress test is to provide useful information about your baby’s oxygen supply by checking his or her heart rate and how it responds to your baby’s movement. The test might indicate the need for further monitoring, testing or delivery.
Normally, a baby’s heart beats faster when he or she is active later in pregnancy. However, conditions such as fetal hypoxia — when the baby doesn’t get enough oxygen — can disrupt this response.
Your health care provider might recommend a nonstress test if you have:
- A multiple pregnancy with certain complications
- An underlying medical condition, such as type 1 diabetes, heart disease or high blood pressure during pregnancy
- A pregnancy that has extended two weeks past your due date (post-term pregnancy)
- A history of complications in a previous pregnancy
- A baby who has decreased fetal movements or possible fetal growth problems
- Rh (rhesus) sensitization — a potentially serious condition that can occur, typically during a second or subsequent pregnancy, when your red cell antigen blood group is Rh negative and your baby’s blood group is Rh positive.
- Low amniotic fluid (oligohydramnios)
Your health care provider might recommend having nonstress tests once or twice a week — and occasionally daily — depending on your health and your baby’s health.
Risks
A nonstress test is a noninvasive test that poses no physical risks for you or your baby. The term “nonstress” refers to the fact that nothing is done to place stress on the baby during the test.
While a nonstress test can offer reassurance about your baby’s health, it can cause anxiety, too. A nonstress test might suggest that a problem exists when there is none, which can lead to further testing. Reassuring results also aren’t predictive of the future.
Also, keep in mind that while a nonstress test is often recommended when there’s an increased risk of pregnancy loss, it’s not always clear if the test is helpful.
How you prepare
A nonstress test typically requires no special preparation.
What you can expect
A nonstress test is usually done in your health care provider’s office.
Before the procedure
You’ll have your blood pressure taken before the nonstress test begins.
During the procedure
During the nonstress test, you’ll lie on a reclining chair. You’ll have your blood pressure taken at regular intervals during the test.
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.
Typically, a nonstress test lasts 20 minutes. However, if your baby is inactive or asleep, you might need to extend the test for another 20 minutes — with the expectation that your baby will become active — to ensure accurate results. Your health care provider might try to stimulate the baby by placing a noise-making device on your abdomen.
After the procedure
After the nonstress test is complete, your health care provider will likely discuss the results with you right away.
Results
Results of a nonstress test are considered:
- Reactive. Before week 32 of pregnancy, results are considered normal (reactive) if your baby’s heartbeat accelerates to a certain level above the baseline twice or more for at least 10 seconds each within a 20-minute window. At week 32 of pregnancy or later, if your baby’s heartbeat accelerates to a certain level above the baseline twice or more for at least 15 seconds each within a 20-minute window, the results are considered reactive.
- Nonreactive. If your baby’s heartbeat doesn’t meet the criteria described above, the results are considered nonreactive. Nonreactive results might occur because your baby was inactive or asleep during the test.
A reactive nonstress test is considered reassuring regardless of the length of observation time needed. However, if the test is extended to 40 minutes and your baby’s nonstress test results are nonreactive, your health care provider will likely do another prenatal test to further check your baby’s health. For example:
- Biophysical profile. A biophysical profile combines a nonstress test with a fetal ultrasound that evaluates your baby’s breathing, body movements, muscle tone and amniotic fluid level.
- Contraction stress test. This test looks at how your baby’s heart rate reacts when your uterus contracts. During a contraction stress test, if adequate uterine activity doesn’t occur on its own you will be given intravenous oxytocin or be asked to rub your nipples to induce uterine activity.
Your health care provider might also ask you to have another nonstress test that day.
Other possible causes of a nonreactive nonstress test result, besides the fetus being inactive or asleep, include decreased oxygen (fetal hypoxia), maternal smoking, maternal use of medications, and fetal neurological or cardiac anomalies.
Rarely, during a nonstress test, problems with a baby’s heart rate are detected that require further monitoring or treatment.
Be sure to discuss the results of your nonstress test with your health care provider and what they might mean for you and your baby.
The Fetal Non-Stress test is a simple, non-invasive test performed in pregnancies over 28 weeks gestation. The test is named “non-stress” because no stress is placed on the fetus during the test.
How is an NST Performed?
The test involves attaching one belt to the mother’s abdomen to measure fetal heart rate and another belt to measure contractions. Movement, heart rate and “reactivity” of heart rate to movement are measured for 20-30 minutes. If the baby does not move, it does not necessarily indicate there is a problem; the baby could just be asleep.
A nurse may use a small “buzzer” to wake the baby for the remainder of the test.
Why would an NST be performed?
An NST may be performed if:
- You sense the baby is not moving as frequently as usual
- You are overdue
- There is any reason to suspect the placenta is not functioning adequately
- You are high risk for any other reason
The test can indicate if the baby is not receiving enough oxygen because of placental or umbilical cord problems; it can also indicate other types of fetal distress.
Are there any risks or side effects?
An NST is a noninvasive test that poses no known risks or side effects to mother or baby.
When is an NST performed?
NSTs are generally performed after 28 weeks of gestation. Before 28 weeks, the fetus is not developed enough to respond to the test protocol.
What does the NST look for?
The primary goal of the test is to measure the heart rate of the fetus in response to its own movements. Healthy babies will respond with an increased heart rate during times of movement, and the heart rate will decrease at rest. The concept behind a non-stress test is that adequate oxygen is required for fetal activity and heart rate to be within normal ranges.
When oxygen levels are low, the fetus may not respond normally. Low oxygen levels can often be caused by problems with the placenta or umbilical cord.
What do the NST results mean and what are the reasons for further testing?
A reactive non-stress result indicates that blood flow (and oxygen) to the fetus is adequate. A nonreactive non-stress result requires additional testing to determine whether the result is truly due to poor oxygenation, or whether there are other reasons for fetal non-reactivity (i.e. sleep patterns, certain maternal prescription or nonprescription drugs).
Compiled using information from the following sources:
1. WebMD
https://www.webmd.com
2. American College of Obstetricians and Gynecologists, Special Tests for Monitoring Fetal Health
https://www.acog.org/~/media/For%20Patients/faq098.pdf
New Hanover Health Network, What is a Non-stress Test
