Socioeconomic differences in the association between maternal age and maternal obesity: a register-based study of 707,728 women in Finland.

AIMS: To examine the association between maternal age and maternal obesity across socioeconomic groups and to determine whether socioeconomic status modifies the association between maternal age and maternal obesity with a view to informing public health policies. METHODS: Data for this register-based study were sourced from the Finnish Medical Birth Register and Statistics Finland, using the information of 707,728 women who gave birth in Finland from 2004 to 2015. We used multivariable regression models to assess the association between maternal age and maternal obesity across socioeconomic groups. We further assessed interactions on both multiplicative and additive scales. RESULTS: Across all socioeconomic groups, the adjusted odds ratio for the association between maternal age and maternal obesity increased, peaking for women 35 years or older. Using women below 20 years of age in the category of upper-level employees as a single reference group, in the category of upper-level employees, the adjusted odds ratio and 95% confidence intervals among women 35 years or older was 1.92 (1.39-2.64) for maternal obesity. Equally, the adjusted odds ratio and 95% confidence intervals in the category of long-term unemployed was 4.35 (3.16-5.98). Synergistic interactions on both multiplicative and additive scales were found across age and socioeconomic groups. CONCLUSIONS: The association between maternal age and maternal obesity was strongest among women 35 years or older with lower socioeconomic status. Population-level interventions that address maternal risk factors from teenage years are needed alongside individual-level interventions that target high-risk mothers in areas of low socioeconomic status and maternal obesity.