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

A non-stress test (NST) primarily measures fetal heart rate reactivity in response to fetal movement. During the test, the fetal heart rate is monitored, and the responses to fetal movements are recorded. An increase in heart rate of 15 beats per minute or more for at least 15 seconds in response to movements typically indicates that the fetus is healthy and receiving enough oxygen, as it demonstrates an intact nervous system and the ability to respond appropriately to stimuli.

This assessment is crucial, especially in high-risk pregnancies where monitoring fetal well-being is essential. It helps healthcare providers determine if further testing or intervention is required to ensure the safety of both the mother and the fetus. Thus, the focus on fetal heart reactivity is why this option is the correct answer.