When a primigravida's membranes rupture spontaneously, the priority nursing action should be to?

In the situation where a primigravida's membranes rupture spontaneously, assessing the fetal heart rate is the most critical nursing action. This is because the rupture of membranes can lead to a number of changes in the fetal condition, and monitoring the fetal heart rate allows the healthcare provider to assess how the fetus is tolerating the event.

Once the membranes have ruptured, there's an increased risk of umbilical cord compression or fetal distress, which can be identified through changes in the fetal heart rate patterns. This assessment is vital to ensure that the fetus is not in distress and that there are no complications arising from the rupture of membranes.

After ensuring the fetal heart rate is stable, other important assessments, such as monitoring the contractions, checking maternal vital signs, or documenting the characteristics of the amniotic fluid, can be addressed. However, the immediate concern is the well-being of the fetus, making the assessment of the fetal heart rate the priority action.