Non-Stress Test (NST) is a simple, non-invasive, prenatal test conducted to assess the health of the baby after 28 weeks of pregnancy. NST monitors the heart rate of the fetus as it moves about in the womb along with several other parameters. Fetal non-stress test is so named, as it does not cause stress to the baby in any way during the test.
The heartbeat of the fetus changes during the course of pregnancy. The heartbeat of the fetus is faster when the fetus is active and moves about in the later stages of pregnancy. NST during pregnancy is used to obtain information about the baby's oxygen supply by checking his heart rate and change in it as a reaction to movement.
Non stress test interpretation is categorized as reactive (normal) or nonreactive.
Reactive NST: A non-stress test is considered normal when it is reactive. This means that the heart rate of the baby increased when it moved in the uterus. Before 32 weeks of pregnancy, an increase in heartbeat baseline up to a certain specified level at least twice, for >/= 10 seconds, in a 20-minute period is considered reactive.
After 32 weeks of pregnancy, the heartbeat should increase to a specified level above baseline for 15 seconds or more during a 20-minute period at least twice.
Non reactive NST: A non reactive NST implies that the heart rate of the baby did not increase according to the above specified criteria during the test period of 20 minutes. The test is generally extended to 40 minutes duration as it is possible that baby was asleep. Also, efforts are made to wake up the sleeping baby.
In case, it is still non-reactive, the doctor may prescribe another prenatal test to check on the health of the fetus. In cases where the pregnancy is full term (39 weeks or more), the doctor may even recommend an early delivery.
Remember that a non-reactive NST simply indicates that the heart rate of the baby did not increase during the period of the test. It is simply an indicator for activity during a particular period and should not be considered as a confirmed diagnosis of fetal sickness. A reactive NST on the other hand, is considered much more reliable as an indicator of wellbeing of the baby.
A non reactive NST indicates that the baby was not so active or was asleep during the test. However, when this is not the case, it indicates that the blood flow and the oxygen levels of the fetus are not adequate (Fetal hypoxia). Factors that contribute to fetal hypoxia and can cause non reactive result are smoking by the expectant mother, use of certain medications or drugs, and neurological or cardiac abnormalities in the fetus.
The doctor may recommend that NST be conducted on a regular basis to keep a track on the health of the baby after a non-reactive NST. A biophysical profile or a contraction stress test is generally prescribed to evaluate the health of the baby in greater detail. The doctor may also prescribe other tests to check the health of the baby depending on the results of the NST.
Don’t stress over a non-stress test. It is a way to make amends well in time, if it turns out non-reactive.
Also read: Fetal Heart Rate