Advanced maternal age is not an independent risk factor for low birth weight and preterm delivery: a within-family analysis using Finnish population registers

Alice Goisis, London School of Economics and Political Science (LSE)
Hanna Remes, University of Helsinki
Kieron Barclay, London School of Economics and Political Science (LSE)
Pekka Martikainen, University of Helsinki
Mikko Myrskylä, Max-Planck Institute for Demographic Research and London School of Economics and Political Science (LSE)

Advanced maternal age (35 and over) at birth is considered a major risk factor for birth outcomes. However, 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 negative birth outcomes, we compare between-family (comparing children born to different mothers at different ages) and within-family (comparing children that are born to the same mother at different ages) models. The latter approach reduces confounding by unobserved parental characteristics that are shared by siblings and has never been applied to study the association between advanced maternal age and birth outcomes. We use Finnish population registers of children who were born between the years 1987-2000 (n=124,098). As birth outcomes, we consider being born low birth weight (less than 2500 g) and/or preterm (less than 37 weeks of gestation). The between-family models reproduce the results that are common in the literature and document a robust association between advanced maternal age and the risk of LBW – even after adjustment for observable parental characteristics. Giving birth in the age group 35-39 increases the probability of low birth weight by 1.1% (95% CI: 0.008-0.014) and in the group 40 and above by 2.2% (95% CI: 0.014 to 0.029). In contrast, the within-family models – which reduce confounding of unobserved parental characteristics shared by siblings - show that this association is negligible both statistically and substantively. Results for preterm are qualitatively similar. The results suggest that an advanced maternal age is not an independent risk factor for low birth weight and preterm among mothers who have at least two live births. This finding is of great relevance both for women who are contemplating the postponement of childbearing and for physicians who are providing advice to patients about these risks.