Rupture of fetal membranes can occur when the cervix is either closed or dilated. Sometimes, it can occur in a very early pregnancy (before 28 weeks – this leads to inevitable abortion, which you will learn about in Study Session 20), or in early third trimester (between 28 and 34 weeks). It is not exactly known why fetal membranes rupture before the onset of labour. However, there are some known risk factors highly associated with PROM.
Consider the amniotic cavity as a sac (or bag) whose wall is formed by the fetal membranes, enclosing the fetus and amniotic fluid. The sac will rupture at the weakest point, which is the part of the membranes in direct contact with the 'mouth' of the cervix. Rupture happens when the sac is either damaged by an infection or external trauma, or it becomes over-stretched (distended) and unable to withstand the internal pressure. These risk factors are described in more detail below.