Final answer:
The first action when a woman's membranes spontaneously rupture during the second stage of labor is to assess the fetal heart rate to ensure the baby's health.
Step-by-step explanation:
When a woman's membranes spontaneously rupture during the second stage of labor, the first action to take is to assess the fetal heart rate. This is a critical step to ensure the wellbeing of the baby, as changes in the heart rate can indicate distress. Documenting the time of rupture is also important, but it follows the immediate assessment of the fetal health. Administering pain medication and encouraging pushing are subsequent considerations after ensuring fetal safety and depending on the stage of labor and condition of the mother.