Methods and results

We obtained our information from the Aberdeen Maternal and Neonatal Databank for all singleton deliveries to city residents aged at least 20 years during 1988-97; totalling 23 806 deliveries.3 We used logistic regression to obtain crude odds ratios for delivery by caesarean section among older women (age categories 30-31, 32-33, 34-35, 36-37, 38-39, and 40 and over) compared with a reference group of women aged 20-29 years. Primiparous and multiparous women were analysed separately, as were elective and emergency caesarean sections. We investigated the potential confounders of the association between age and outcome, and we also checked for any evidence of effect modification with the same variables—maternal sociodemographic characteristics and obstetric history.

We selected the obstetric complications and interventions associated with a higher probability of caesarean section, which might explain the association with age (see figure on web). Using multivariate logistic regression we adjusted the crude odds ratios for these variables, also controlling for any identified confounders and stratifying by effect modifiers.

The association between maternal age and caesarean section varied depending on how the baby presented at delivery and whether a woman had previously had a caesarean. Among women who had not previously had a caesarean section and whose babies presented normally at delivery there was a strong and consistent relation between maternal age and delivery by caesarean section that remained after controlling for relevant obstetric complications and identified confounders (see figure on web). Among women who had had a previous caesarean section or whose babies presented abnormally the association between maternal age and both elective and emergency sections was greatly reduced (results not shown).

Comment

The observed relation between maternal age and caesarean section cannot be explained by the obstetric complications we considered. They add to previous findings by including greater numbers of obstetric complications and deliveries in the analysis. This raises the question of why rates for caesarean section are high among older mothers, and whether they may be explained by physiological or other factors we have been unable to control for. Medical causes that have been suggested include reduced uterine function and pelvic compliance among older women.4 However we expected that length of labour would act as a proxy for these factors in our analysis. Many authors have identified physician and maternal preference in the higher section rates among older women, 1 5 and our results would support this speculation. Further investigation is needed into women's views about increased intervention, the variation in rates for caesarean section among obstetricians, and how maternal age influences both of these factors.