BACKGROUND: In Australia, birth rates for women aged 35 years or more are significant and increasing and a considerable percentage are first births. This study investigated the effect of maternal age on interventions in labor and birth for primiparous women aged 35 to 44 years compared with primiparous women aged 25 to 29 years. METHODS: All primiparous women who gave birth in Victoria, Australia, in 2005 and 2006 (n = 57,426) were included in this population-based cross-sectional study. Women were stratified by admission status (private/public). Main outcome measures were induction of labor, augmentation of labor, use of epidural analgesia, and method of birth. Multivariate logistic regression was used to explore the relationship between maternal age and cesarean adjusted for confounders. RESULTS: Older women were more likely to give birth by cesarean section whether admitted as public or private patients. For private patients, total cesarean rates were 31.8 percent (25-29 yr), 46.0 percent (35-39 yr), and 60.0 percent (40-44 yr; p < 0.001) compared with 27.5, 41.6, and 53.4 percent for public patients (p < 0.001). Older women who experienced labor were more likely to have an instrumental vaginal birth or an emergency cesarean section than younger women. Both were more common in women admitted as private patients. Age-related trends were also seen for induction of labor and use of epidural analgesia. Rates were higher for private patients. Rates of induction were (37.8, 40.2, and 42.5%) for private patients compared with (32.1, 36.7, and 40.1%) for public patients and rates for epidural were (45.3, 49.9, and 48.1%) among private patients compared with (33.3, 38.8, and 39.3%) among public patients. CONCLUSIONS: Interventions in labor and birth increased with maternal age, and this effect was seen particularly for cesarean section among women admitted privately. These findings were not fully explained by the complications we considered.