A fetal nonstress test is a way to measure the heart rate of the fetus during the third trimester. It is usually only given after week 28 of pregnancy, and is considered noninvasive, as it does not place any stress on the mother or baby. It typically consists of two belts strapped around the mother's waist, with one monitoring her contractions and the other monitoring the baby's heart rate as he moves. A slow heart rate in the fetus during movement may indicate a lack of oxygen, which tells doctors that something may be wrong and should be remedied quickly.
This kind of test is usually only performed after week 28 since babies are not developed enough to react to the nonstress test before this period of gestation. The test is usually ordered by a doctor when the mother notices decreased movement in the fetus, is considered high risk, or is overdue. In general, if there is any reason to believe that the baby is not getting enough oxygen, a nonstress test will be given. Fortunately, it is not invasive, as the two belts simply wrap around the mother's waist for anywhere from 20 minutes to an hour.
During the nonstress test, both the fetal heart rate and the mother's contractions are graphed on a computer monitor. The goal is to see that the baby's heart rate increases with each of his movements, as this indicates that the oxygen levels are adequate. A baby whose heart rate is within the normal range of 120 to 160 beats per minute is said to have a reactive nonstress test, while a baby whose heart rate seems to stay the same during movement has a nonreactive nonstress test. This may be caused by problems with the placenta or umbilical cord, through which the baby receives its oxygen. On the other hand, it may mean that the baby is asleep, in which case he can usually be woken up by the mother drinking a sweet drink; sometimes the doctor may also use a loud buzzer to wake the baby.
A test of this sort can be offered in the doctor's office, though it is most often given in the emergency room or labor and delivery room when there is a suspected problem with the fetus. In some cases, this test is done when an otherwise normal pregnancy continues past the due date, as it is important to monitor overdue babies to ensure that they are not in distress. Fortunately, the only danger of having a nonstress test is misinterpreting data, which makes it one of the safest tests offered.