Effect of changes in maternal age, parity, and birth weight distribution on primary cesarean delivery rates.

OBJECTIVE

To examine the effect of maternal age on cesarean delivery risk and to quantify the impact of demographic changes since 1970 on primary cesarean delivery rates.

DESIGN

A cohort study.

SETTING

Nonfederal short-stay hospitals in Washington State.

PARTICIPANTS

All women who delivered live singletons with linked birth certificate and hospital discharge data from 1987 through 1990.

MAIN OUTCOME MEASURES

Maternal age-, birth weight-, and parity-specific primary cesarean delivery rates, Mantel-Haenszel relative risk estimates for primary cesarean delivery by 5-year age category stratified by parity, and direct standardization of 1987 through 1990 primary cesarean rates to 1970 Washington State maternal age, birth weight, and parity distribution.

RESULTS

Primary cesarean rates ranged from 3.2% for multiparous teenage women who delivered infants weighing 3500 g through 3999 g to 58.9% for primiparous women 40 years of age or older who delivered infants weighing 4000 g or more. After adjustment, the risk of cesarean delivery increased with each 5-year age increment among women 20 years of age or older. We estimated that if the maternal age, parity, and birth weight distribution from 1987 through 1990 were identical to what existed in 1970, Washington State's primary cesarean rate from 1987 through 1990 would have been 12.2%, compared with the observed rate of 14.8%.

CONCLUSION

The lower adjusted primary cesarean rate reflects the demographic changes in the childbearing population, which may be responsible for 18% of the 1987 through 1990 cesarean delivery rates. These findings suggest the importance of using maternal age-, birth weight-, and parity-specific primary cesarean delivery rates to compare populations and study temporal trends.