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 p rimary cesarean delivery rates. Design.-A cohort study. Setting.-Nonfe deral short-stay hospitals in Washington State. Participants.-All wome n who delivered live singletons with linked birth certificate and hosp ital discharge data from 1987 through 1990. Main Outcome Measures.-Mat ernal age-, birth weight-, and parity-specific primary cesarean delive ry rates, Mantel-Haenszel relative risk estimates for primary cesarean delivery by 5-year age category stratified by parity, and direct stan dardization of 1987 through 1990 primary cesarean rates to 1970 Washin gton State maternal age, birth weight, and parity distribution. Result s.-Primary cesarean rates ranged from 3.2% for multiparous teenage wom en who delivered infants weighing 3500 g through 3999 g to 58.9% for p rimiparous women 40 years of age or older who delivered infants weighi ng 4000 g or more. After adjustment, the risk of cesarean delivery inc reased with each 5-year age increment among women 20 years of age or o lder. 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%. Concl usion.-The lower adjusted primary cesarean rate reflects the demograph ic changes in the childbearing population, which may be responsible fo r 18% of the 1987 through 1990 cesarean delivery rates. These findings suggest the importance of using maternal age-, birth weight-, and par ity-specific primary cesarean delivery rates to compare populations an d study temporal trends.