External fetal monitors primarily measure which of the following?

External fetal monitors are primarily designed to measure the frequency, regularity, and duration of uterine contractions. This is crucial during labor, as monitoring these parameters helps healthcare providers assess the progress of labor and the well-being of the fetus. By detecting the contraction patterns, the monitor provides valuable information that aids in managing labor effectively.

While external fetal monitors do provide some insight into the fetal heart rate and can be used in conjunction with other methods to evaluate the overall condition of the fetus, their primary function focuses on uterine contractions. This monitoring is essential in identifying any abnormal patterns that may indicate distress for either the mother or the fetus during childbirth.