Outcome
Several outcome variables of preterm birth were created
based on completed weeks of gestation: very preterm birth
(22–31 weeks of gestation); and moderately preterm birth (32–36 weeks of gestation). Spontaneous preterm birth was
defined as being born preterm after a spontaneous onset of
labour (as recorded on the delivery record) or having a
diagnosis of preterm prelabour rupture of membranes
(ICD–10 code O42). Medically indicated preterm birth was
defined as being born preterm and having an induced
onset of labour or a caesarean section before the onset of
labour.