Final answer:
The first thing to check when membranes rupture is the expulsion stage of labor.
Step-by-step explanation:
When membranes rupture, the first thing you should check is the expulsion stage. The amniotic membranes in pregnant women rupture at the end of the dilation stage in response to excessive pressure from the fetal head entering the birth canal. This is commonly known as the 'water breaking' during labor.