Primary and Repeat Cesarean Deliveries: A Population-based Study in the United States, 1979-2010

Despite the temporal increase in cesarean deliveries, the extent to which maternal age, period, and maternal birth cohorts may have contributed to these trends remains unknown.

We performed an analysis of 123 million singleton deliveries in the United States (1979-2010). We estimated rate ratio (RR) with 95% confidence interval (CI) for primary and repeat cesarean deliveries. We examined changes in cesarean rates with weighted Poisson regression models across three time-scales: maternal age, year of delivery, and birth cohort (mother's birth year).

The primary cesarean rate increased by 68% (95% confidence interval [CI]: 67%, 69%) between 1979 (11.0%) and 2010 (18.5%). Repeat cesarean deliveries increased by 178% (95% CI: 176, 179) from 5.2% in 1979 to 14.4% in 2010. Cesarean rates increased with advancing age. Compared with 1979, the RR for the period effect in primary and repeat cesarean deliveries increased up to 1990, fell to a nadir at 1993, and began to rise thereafter. A small birth cohort effect was evident, with women born before 1950 at increased risk of primary cesarean; no cohort effect was seen for repeat cesarean deliveries. Adjustment for maternal BMI had a small effect on these findings. Period effects in primary cesarean were explained by a combination of trends in obesity and chronic hypertension, as well as demographic shifts over time.

Maternal age and period appear to have important contributions to the temporal increase in the cesarean rates, although the effect of parity on these associations remains undetermined.