Before a baby is born, the amniotic sac breaks open, causing amniotic fluid to either leak slowly or gush out. When this happens before contractions start, it is called prelabour rupture of membranes (PROM).

PROM can occur at any time during pregnancy before labour begins. Early PROM (before 37 completed weeks of pregnancy) may be referred to as preterm prelabour rupture of membranes, or pPROM.

PROM is typically unexpected, and the cause is often difficult to identify. Known causes of PROM include uterine infection; overstretching of the uterus, such as by twins or more or by an excess of amniotic fluid; and trauma, such as from a car crash.

Labour usually begins shortly after PROM occurs. If PROM occurs after 34 to 36 weeks of pregnancy and labour does not start within 12 to 18 hours, labour may be induced to reduce the risk of infection.