This study aimed to determine if fetal bacteraemia and amniotic fluid infection at the time of membrane rupture reduces the interval between membrane rupture and the onset of labour in pregnancies complicated by preterm prelabour amniorrhexis. Sixty nine pregnancies with preterm prelabour amniorrhexis at 12-36 weeks' gestation that were managed expectantly had spontaneous onset of labour. In all cases cordocentesis and amniocentesis were performed and fetal blood and amniotic fluid were cultured for aerobic and anaerobic bacteria. In the group with negative fetal blood and amniotic fluid cultures (group 1) the median interval from amniorrhexis to delivery was 41 days (range 1-161) and there was an inverse correlation between gestational age at amniorrhexis and delivery interval. In the group with negative fetal blood but positive amniotic fluid cultures (group 2) the median amniorrhexis to delivery interval was nine days (range 1-37), and in the group with positive fetal blood cultures (group 3) the interval was two days (range 1-5). These findings suggest that pregnancies complicated by preterm prelabour amniorrhexis and fetal bacteraemia undergo spontaneous labour within five days of membrane rupture, and if labour does not occur then infection is unlikely.