What does a non-stress test (NST) measure?

A non-stress test (NST) primarily measures the fetal central nervous system (CNS) activity by observing the fetal heart rate in response to fetal movements. The test is designed to assess how well the baby's heart handles movement and to evaluate the overall well-being of the fetus. During the NST, if the fetal heart rate increases with movement—indicating that the CNS is intact and responsive—it is generally considered a reassuring sign that the fetus is healthy.

The focus on fetal CNS activity is significant because a healthy reaction in the fetal heart rate suggests adequate oxygenation and an appropriate response from the CNS, essential for fetal development. The NST does not directly measure maternal heart rate, growth patterns of the fetus, or levels of maternal stress. Instead, its primary intent is to provide insights into the fetal condition, especially in high-risk pregnancies where fetal monitoring is crucial.