A fetal Non-Stress test (NST) 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 baby during the test. The test involves attaching one belt to the mother’s abdomen to measure the fetal heart rate and another belt to measure the contractions. The test can take as little as 20-30 minutes or as long as an hour.
The test measures fetal movement, heart rate, and how reactive the baby’s heart rate is to movement. 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.
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. An NST test is a low-risk, painless, and non-invasive procedure. The biggest risk during an NST is a misinterpretation of the data.
Always share any concerns with your provider before undergoing the test. He or she can reassure you about steps being taken to prevent any risk to you or your baby. Generally, NSTs are performed after 28 weeks of gestation because before 28 weeks, the fetus is not developed enough to respond to the test protocol.
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.
The result of an NST is either reactive or nonreactive. A reactive non-stress is when the fetal heart rate goes up at least twice or more during the testing period. This is a normal result, indicating the baby is healthy and that the blood flow (and oxygen) to the fetus is adequate.
A nonreactive non-stress result is when the fetal heart rate does not change when the baby moves. The provider may take steps to stimulate the baby and perform the NST test again, or he or she may order additional tests 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).
If your baby is not as responsive as expected during the fetal non-stress test, your provider may order additional tests, including a biophysical profile (BPP) or a pregnancy stress test, to gather more information.
The BPP includes a highly detailed ultrasound that allows the technician to measure and assess certain factors, including the baby’s breathing, muscle tone, and movement. Depending upon the BPP results, the provider may order more tests or move forward with a cesarean section or induction.
A pregnancy stress test (also called a contraction stress test) is another tool to assess fetal well-being, specifically how the baby responds to contractions. This test is usually done in the hospital, during late pregnancy, or even in the early stages of labor. During a pregnancy stress test, a drug called Pitocin is administered intravenously to initiate contractions. The baby’s heart rate is monitored throughout the process. If the baby passes the stress test, you can usually wait for labor to begin on its own. Otherwise, it may be time for a cesarean section or induction.