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| |    Non-stress Testing by Marjorie Greenfield, M.D. reviewed by Marjorie Greenfield, M.D. Non-stress testing (NST) simply involves monitoring a developing baby's heart rate over time. The monitor has two belts that go around the mother's waist-one registers any contractions she may have, the other tracks the fetal heart rate. These are graphed on paper or on a computer screen. Often, you can see the graph as it is being made. Usually, the NST is not uncomfortable, although it may get tiresome to stay in one position for 20 minutes or so (in fact, it's not unusual for these tests to run as long as an hour). Reactive vs. nonreactive tests The usual baseline fetal heart rate is between 120 and 160 beats per minute. Once the monitor is in place, your practitioner will look for certain measurements to see how the baby is faring, including if his heart rate rises when he moves. An NST is considered reassuring if there are accelerations of the fetal heart rate of at least 15 beats per minute over the baseline, lasting at least 15 seconds, occurring within a 20-minute time block. This is called a reactive NST. If these accelerations don't occur, the test is said to be nonreactive. In addition, since many women have mild contractions that they may not even notice, your practitioner will note any of the baby's responses to contractions or if the fetal heart rate dips below baseline. What does it mean if the NST is nonreactive? A fetus who is sleeping may not demonstrate accelerations. Your practitioner will have to wait, then, for him to wake up. Although a reactive NST is a good sign, a nonreactive NST does not mean the baby is in trouble. If you and your practitioner aren't reassured by the results of the NST, or if the fetal heart rate slows down alarmingly, more testing is usually done. This might include a more prolonged NST, a contraction stress test, or a biophysical profile.
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