Objective: To study the consequences of delayed first childbearing in a large, population-based US sample, with separate analysis of women aged 40 years or more and adjustment for socioeconomic factors, smoking, medical and reproductive conditions, and route of delivery.
Design and setting: Retrospective survey of Washington State birth certificates from 1984 through 1988.
Subjects: First liveborn singleton infants of women aged at least 20 years. Of eligible white infants, all those born to women aged 35 to 39 years (n = 4019) and 40 years or more (n = 410) and a maternal age-stratified random sample of white infants of younger women were studied. All eligible black infants were studied.
Outcome measures: Low (< 2500 g) and very low (< 1500 g) birth weight and preterm delivery (< 37 weeks of gestation).
Results: Adjusted odds ratios for delivering a low-birth-weight white infant increased progressively with each 5-year maternal age group, reaching 2.3 (95% confidence interval, 1.6 to 3.4) for women aged 40 years or more compared with those aged 20 to 24 years. The maternal age effects for very low birth weight and preterm delivery were similar; for each, the odds ratio was 1.8 for the oldest group. No significant maternal age effect was found among births of black infants, but only 127 births to women aged 35 years or more were studied.
Conclusion: Increasing maternal age at first childbirth is an independent risk factor for low birth weight and preterm delivery of white infants in the United States.