Paternal age and the risk of low birth weight and preterm delivery: a Finnish register-based study

Based on existing studies, there is no conclusive evidence as to whether and why paternal age matters for birth outcomes.

We used Finnish population registers on 106 652 children born 1987-2000. We first document the unadjusted association between paternal age and the risk of low birth weight (LBW; <2500 g) and preterm birth (<37 weeks' gestation). Second, we investigate whether the unadjusted association is attenuated on adjustment for child's, maternal and parental socioeconomic characteristics. Third, by adopting a within-family design which involves comparing children born to the same father at different ages, we additionally adjust for unobserved parental characteristics shared between siblings.

The unadjusted results show that being born to a father aged 40+, as opposed to a father aged 30-34, is associated with an increased risk of LBW of 0.96% (95% CI 0.5% to 1.3%) and to a younger father (<25) with a 1% (95% CI 0.6% to 1.3%) increased risk. The increased risk at younger paternal ages is halved on adjustment for the child's characteristics and fully attenuated on adjustment for child/parental characteristics. The increased risk at paternal ages 40+ is partially attenuated on adjustment for maternal characteristics (β=0.62%; 95% CI 0.13% to 1.1%). Adjustment for unobserved parental characteristics shared by siblings further attenuates the 40+ coefficient (β=0.4%; 95% CI -0.5% to -1.2%). Results for preterm delivery are similar.

The results underscore the importance of considering paternal age as a potential risk factor for adverse birth outcomes and of expanding research on its role and the mechanisms linking it to birth outcomes.