The non- stress test refers to the fact that no medication is given to the mother to cause movement of the fetus or contraction of the uterus. It is often used to confirm the well-being of the fetus based on the principle that a healthy fetus will demonstrate an acceleration in its heart rate following movement. Fetal activity may be spontaneous or induced by external manipulation such as rubbing the mother's abdomen or making a loud noise above the abdomen with a special device. When movement of the fetus is noted, a recording of the fetal heart rate is made. If the heart rate goes up, the test is normal.
Labor represents a stress to the fetus.
Every time the uterus contracts, the fetus is momentarily deprived of its usual blood supply and oxygen. This is not a problem for most babies. However, some babies are not healthy enough to handle the stress and demonstrate an abnormal heart rate pattern. This test is often done if the non-stress test is abnormal. The contractions are a challenge to the baby, similar to the challenge of normal labor.