Delayed childbearing and its impact on population rate changes in lower birth weight, multiple birth, and preterm delivery

Objective. This study quantified the impact of delayed childbearing (maternal age greater than or equal to 35 years) on population rate changes in low birth weight (LBW; <2500 g), preterm delivery (<37 weeks), multiple births, and small for gestational age (SGA; <10th percentile) in Alberta, Canada, between 1990 (N = 42 930) and 1996 (N = 37 710). Methods. Data were obtained from the provincial notification of a live or stillbirth. Analyses included relative risk estimates and χ(2) tests for trend. Potential confounding attributable to in vitro fertilization was investigated. Results. The proportion of births to women greater than or equal to 35 years of age was 8.4% in 1990 and 12.6% in 1996, a 51.2% increase. Among these women, LBW delivery increased 11%, and preterm delivery increased 14%. Delayed childbearing accounted for 78% of the change in LBW rate in the population and 36% of the change in preterm delivery rate in the population. Provincial multiple birth rates increased by 15% for twins and 14% for triplets. Delayed childbearing accounted for 15% of the twin increase and 69% of the triplet increase. When in vitro fertilization pregnancies were excluded, the change was 43% for preterm rates, 100% for LBW, 14% for twins, and 9% for triplets. Delayed childbearing did not contribute to changes in singleton SGA deliveries. Conclusions. The findings suggest that the recent increase in LBW and preterm delivery is partly related to the population phenomenon of delayed childbearing. Maternal age was not related to changes in SGA, suggesting that the age effect is through pregnancy complications that lead to preterm delivery and LBW. Prospective parents should be informed about the higher risk for neonatal morbidity associated with delayed childbearing. Health care providers should be aware of the impact of delayed childbearing on health care resources.