A “nonstress test” is so named because no particular stress is placed on the fetus by performing such a test. Whether the mother feels stressed by it depends on her anxiety level. A nonstress test involves attaching a belt with fetal heart rate and uterine contraction monitors around the mother's abdomen. The heart rate is recorded for approximately 20-30 minutes, during which time the mother indicates whether she feels any fetal movements. This test has been performed in literally millions of women, and fetal heart rate patterns associated with normal outcomes in the baby have been identified. These patterns include: heart rate in the normal range (approximately 110 to 160 beats per minute), presence of occasional brief increases or accelerations in fetal heart rate (generally associated with fetal movements), and a lack of pronounced drops (decelerations) in fetal heart rate. These findings constitute what is called a “reactive” nonstress test, as opposed to a “nonreactive” nonstress test that would be characterized primarily by a lack of fetal heart rate accelerations.
The idea behind a nonstress test is that proper amounts of oxygen are required for the brain to send signals that will be transmitted via nerves to the heart, signals to which the heart will respond appropriately. When oxygen levels are low, the brain, nerves, and/or heart may not respond normally, and the resulting fetal heart rate patterns will not be reactive. It is important to note that many other factors besides lack of oxygen may cause a so called “nonreactive” pattern: fetal sleep cycles, certain maternal prescription or non-prescription drugs, and prematurity are among the most common of these.
A reactive nonstress test is a good predictor of adequate fetal oxygenation, and most reactive fetuses do well for at least another week. A nonreactive nonstress test requires further assessment to determine whether nonreactivity truly is due to decreased oxygen, or whether it may be due to one of the previously mentioned causes.