The aetiology of preterm labour.

Factors associated with spontaneous preterm birth were analysed in 254 primigravidae who gave birth to singleton infants between 28 and 36 completed weeks gestation in Aberdeen City District between 1978 and 1982. In 144 women labour followed premature rupture of the membranes and in 110 the membranes were intact and labour began with contractions. Among those with intact membranes, unmarried women and teenagers were over represented to a significant extent compared with all primigravidae. Among those with premature rupture of the membranes, the distribution of age and marital state was similar to that in all primigravidae, but the birthweight centile distribution showed a significant shift towards light-for-dates babies. It is argued that there may be two distinct categories of pregnancies that end in spontaneous preterm labour.