Objective The risk of operative delivery at term increases linearly with age at first birth. It has been hypothesised that this is because of a deleterious effect of a prolonged interval between menarche and first birth on uterine function. The aim of this study was to test a prediction from the hypothesis, namely, that the risk of operative first delivery would decline with later age at menarche.
Design Retrospective analysis of a prospective cohort study.
Setting The ALSPAC prospective cohort study enrolled pregnant women resident in Avon, UK with expected dates of delivery from 1 April 1991 to 31 December 1992.
Population A total of 3739 primipara recruited to the ALSPAC cohort who experienced labour at term with a singleton infant in a cephalic presentation.
Main outcome measure Operative delivery, defined as caesarean section or operative vaginal birth.
Result The rate of operative delivery was highest among women with age at menarche in the bottom quartile (32.4%, menarche aged ≤12) and was lower in the second (30.3%, menarche aged 13), third (29.2%, menarche aged 14) and top (26.9%, menarche aged ≥15) quartiles (test for trend, P = 0.01). When adjusted for height, body mass index, marital status, smoking status, induction of labour, week of gestation of delivery and birthweight percentile; the odds ratio for operative delivery associated with a 5-year increase in age at menarche (0.78, 95% CI 0.61–0.99) was very similar to the odds ratio for a 5-year decrease in age at delivery (0.73, 95% CI 0.67–0.79). There was no association between age at menarche and the risk of operative delivery following adjustment for the interval between menarche and the first birth (adjusted odds ratio 0.98, 95% CI 0.77–1.25).
Conclusion Later menarche is associated with a decreased risk of operative delivery by decreasing the interval between menarche and first birth. The observation is consistent with the hypothesis that prolonged hormonal stimulation of the uterus prior to the first birth has a deleterious effect on uterine function.