What Is an NST (Nonstress Test)? When and Why Is It Performed During Pregnancy?

During pregnancy, regular monitoring of both maternal and fetal health is important. One of the tests used in this follow-up process is the NST, or nonstress test, a fetal monitoring test.

What Is an NST?

An NST is a painless, noninvasive test used to evaluate the baby’s heart rate, fetal movements and uterine contractions during pregnancy. Its main purpose is to assess fetal well-being and how the fetal heart rate responds to movement.

In a healthy fetus, short accelerations in heart rate are expected during movement. NST helps the physician assess whether these accelerations are adequate during pregnancy follow-up.

When Is an NST Performed?

An NST is usually performed after 32 weeks of pregnancy. Before this period, fetal nervous system maturity and sleep-wake cycles may limit interpretation. In some special situations, however, the physician may request monitoring earlier.

• High-risk pregnancies: Hypertension, gestational diabetes/diabetes, multiple pregnancy, fetal growth restriction or chronic maternal diseases
• Decreased fetal movements perceived by the mother
• Prolonged pregnancy or need for assessment after 40-41 weeks
• History of certain pregnancy losses, preterm birth or high-risk pregnancy
• When needed, Doppler ultrasound may also be used to assess blood flow between the mother, placenta and baby.

What Is the NST Process? How Long Does It Take?

The test often takes approximately 20 to 40 minutes. The pregnant patient usually rests in a side-lying or semi-reclined position. Two belts are placed around the abdomen; one records the baby’s heart rate, and the other measures uterine contractions.

In some settings, the mother is given a button and asked to press it when the baby moves. This allows fetal movements and heart rate changes to be evaluated together. If the baby is in a sleep period, the test may take longer or the physician may want to repeat it.

What Do NST Results Mean?

NST results are generally evaluated in two categories:

• Reactive NST: If the baby’s heart rate accelerates as expected with movement, this is generally considered a reassuring and positive finding.
• Nonreactive NST: If the expected heart rate accelerations are not observed, the test may be repeated or additional evaluation may be performed. A nonreactive result does not always mean the baby is unwell; the baby may be asleep or the monitoring period may have been insufficient.

When needed, the physician may request a biophysical profile, Doppler ultrasound or other fetal assessment tests. NST results should be interpreted together with gestational age, maternal conditions, fetal movements and ultrasound findings.

Is an NST Harmful?

NST is considered safe for the mother and baby. It is noninvasive and does not use radiation. During the test, no medication, needle or radiation is given to the baby. It may be repeated during pregnancy when considered necessary by the physician.

The fact that the test is safe does not mean every pregnant patient needs it at the same frequency. The timing and frequency of NST are determined according to gestational age, risk status and the physician’s clinical assessment.

Are NST Results Available Immediately?

NST results are usually evaluated by the physician immediately after the test. If necessary, the test may be repeated or further evaluation may be performed. Since a nonreactive NST may occur for reasons such as fetal sleep, interpretation should be made by a physician.

Frequently Asked Questions

References

1. Academic Hospital. NST (Non-Stres Test) Nedir? Gebelikte Neden Yapılır?
2. ACOG. Special Tests for Monitoring Fetal Well-Being
3. ACOG. When Pregnancy Goes Past Your Due Date
4. Cleveland Clinic. Nonstress Test (NST): Purpose, Procedure & Results
5. MedlinePlus. Nonstress Test