How many contractions are needed for an adequate contraction stress test (CST)?

An adequate contraction stress test (CST) requires the presence of three contractions that each last a minimum of 40 seconds within a 10-minute observation period. This guideline is established because the goal of the CST is to assess fetal response to uterine contractions, particularly looking for evidence of fetal distress or hypoxia. The three contractions allow for a sufficient assessment of the fetal heart rate in relation to the stimulus provided by the contractions.

This criterion helps ensure that the testing period captures enough data on how the fetus responds to the transient reductions in uteroplacental blood flow that occur with each contraction. The requirement for three contractions specifically balances the need to obtain valid results with the duration necessary for thorough evaluation. The other options either fall short or exceed the established criteria, which define the parameters for an adequate CST. Therefore, the choice indicating three contractions of 40 seconds each aligns correctly with the standard practices utilized in conducting CSTs.