This analysis describes the prevalence of preterm birth by medical decision among 50,307 live births from the district of Seine-Saint-Denis in France, using a classification that distinguishes between medically decided preterm births associated with premature rupture of membranes and those for other reasons. Thirty-seven percent of singleton and 28% of twin preterm births result from labour induction or a caesarean section in the absence of labour. One-quarter of singleton indicated preterm births are associated with premature rupture of membranes. Between 28 and 31 weeks of gestation, 40% of all singleton preterm births result from a medical decision not associated with premature rupture of membranes. The high levels of indicated preterm birth must be taken into account in evaluations of preterm birth rates and trends in developed countries.