Contraction Stress test
Contraction stress testing used to be routine after a nonreactive nonstress test, but in the past two decades largely has been replaced by biophysical profile testing. In a contraction stress test, a fetal heart rate monitor and uterine contraction monitor are placed around the maternal belly, and the fetal heart rate response to contractions is noted. These contractions may occur on their own, through stimulation of the breast, or by administration of medicine through a vein. A normal placenta has extra capacity for transporting oxygen, allowing large amounts of oxygen to pass easily from mother's to fetal blood. If the placenta is damaged, less oxygen may pass. When the womb contracts, the vessels feeding the placenta are squeezed, limiting blood flow and oxygen delivery. If oxygen passage across the placenta drops below a certain point, the fetus responds with a specific type of heart rate deceleration, a decrease in heart rate occurring after the peak of a contraction, called a late deceleration.
The contraction stress test is "positive" if late decelerations occur regularly. A negative CST is one in which no late decelerations occur, and is very reassuring. A positive CST, however, is a sign that the placenta may not be delivering adequate amounts of oxygen to the fetus. Given a positive CST, a pregnancy at term should be delivered, although this is not necessarily every case in very-premature pregnancies. Because the contraction stress test is provocative and takes more time to perform than a biophysical profile, they seldom are done nowadays. There are certain circumstances in which the information from a contraction stress test may be more valuable than that of the biophysical profile, however, particularly in cases of severely preterm pregnancies when low biophysical profile scores may be a "false positives."