What characterizes a positive contraction stress test (CST)?

A positive contraction stress test (CST) is characterized by the presence of late decelerations in the fetal heart rate occurring after 50% or more of the contractions. This finding indicates that the fetus is experiencing stress, typically due to uteroplacental insufficiency. In a healthy fetal environment, contractions would not cause a decline in heart rate; however, late decelerations suggest that the fetus is unable to adequately respond to contractions, raising concerns about its current status and well-being.

Variability in fetal heart rate would typically be seen as a sign of a healthy fetus. The absence of any decelerations can be considered a sign of a reassuring test, indicating that the fetus tolerates contractions well. Fewer than three contractions in ten minutes would suggest a lack of adequate stimulation for the test; therefore, it would not contribute to a positive CST finding. Thus, the presence of late decelerations in relation to contractions is what distinctly characterizes a positive CST.