The maintenance of the structural integrity of the fetal membranes is critical for successful progression of pregnancy. At term rupture of the fetal membranes (ROM) does not occur until the onset of labour-associated contractions in 90–95% of deliveries. This is often referred to as ‘spontaneous’ ROM, but since ‘spontaneous’ is also employed clinically to distinguish ‘natural’ from ‘artificial’ rupture, ‘labour-associated’ ROM (LROM) may be a more appropriate terminology. In the remaining 5–10% of term deliveries the membranes rupture prior to the onset of clinically apparent contractions. This has been referred to as ‘premature’ ROM, but it is less ambiguously defined as ‘pre-labour’ ROM (PROM). In direct contrast to the situation at term, PROM precedes 30–40% of preterm deliveries. If iatrogenic deliveries are excluded, PROM precedes 50% of preterm births, and represents the major cause of preterm birth. There is therefore great interest in the mechanisms underlying membrane rupture whether PROM or LROM.