Advanced maternal age and the risk of low birth weight and preterm delivery: a within-family analysis using Finnish population registers
American Journal of Epidemiology, 186:11, 1219–1226 (2017)
Advanced maternal age at birth is considered a major risk factor for birth outcomes. It is unclear to what extent this association is confounded by maternal characteristics. To test whether advanced maternal age at birth independently increases the risk of low birth weight (<2500g) and preterm birth (<37 weeks gestation), we compare between-family models (comparing children born to different mothers at different ages) with within-family models (comparing children who are born to the same mother at different ages). The latter procedure reduces confounding by unobserved parental characteristics that are shared by siblings. We use Finnish population registers of 124,098 children born 1987-2000. In between-family models, when compared with maternal ages 25-29, maternal ages 35-39 and 40 and above are associated with 1.1 percentage point (95% CI: 0.8,1.4) and 2.2 percentage point (95% CI: 1.4,2.9) increases, respectively, in the probability of LBW. The associations are similar for risk of preterm birth. In within-family models, the relationship between advanced maternal age and LBW or preterm is statistically and substantively negligible. In Finland, advanced maternal age is not independently associated with the risk of LBW and preterm birth among mothers who have at least two live births.