What characterizes a reactive NST for less than 32 weeks?

A reactive non-stress test (NST) for a fetus less than 32 weeks gestation is characterized by accelerations of the fetal heart rate that are significant enough to indicate a healthy response to stimuli. Specifically, accelerations should be defined as increases in heart rate of at least 10 beats per minute (bpm) above the baseline, lasting for a minimum of 10 seconds.

Option B accurately describes this characteristic, as it highlights that the accelerations in heart rate need to exceed the threshold of 10 bpm, signifying an appropriate fetal response and well-being. This response is critical in assessing fetal health, as well as the integrity of placental function, especially in younger fetuses who may have different criteria for reactivity compared to those that are more mature.

The other options reference conditions that do not align with the criteria for a reactive NST. For instance, an absence of accelerations does not indicate reactivity, while decelerations, or extended accelerations lasting longer than 20 seconds, do not meet the specific definitions for a reactive result in this context. Thus, the established characterization of a reactive NST for fetuses under 32 weeks emphasizes the significance of accelerations exceeding 10 bpm, confirming the accuracy of the answer.