Rupture of membranes before the onset of spontaneous labour increases the likelihood of instrumental delivery.

We studied obstetric outcome in 350 consecutive nulliparous women in spontaneous labour and term pregnancy. Women who presented with rupture of membranes before the onset of contractions were more likely to deliver by forceps compared with those in whom contractions preceded rupture of membranes. This increased likelihood of instrumental delivery was significant with and without the use of extradural analgesia in labour (P less than 0.05 and P less than 0.001, respectively). Furthermore, significantly more women with premature rupture of membranes received extradural analgesia in labour (P less than 0.01). We conclude that any study which aims to examine the influence of extradural analgesia on the outcome of delivery should include premature rupture of membranes as a bias factor.