Goisis A, et al. - Using Finnish population registers on 106,652 children born 1987–2000, researchers determined the link between paternal age and the risk of low birth weight (LBW; <2500 g) and preterm birth (<37 weeks’ gestation), adjusting for child’s, maternal and parental socioeconomic characteristics, and unobserved parental characteristics shared between siblings. Being born to a father aged 40+, as opposed to a father aged 30–34, was found to be related to an increased risk of LBW of 0.96% and to a younger father (<25) with a 1% increased risk. Adjustment for the child’s characteristics halved the increased risk at younger paternal ages. Adjustment for maternal characteristics resulted in partial reduction of the increased risk at paternal ages 40+. Further reduction was seen in the 40+ coefficient on adjustment for unobserved parental characteristics shared by siblings. Findings highlight the importance of considering paternal age as a potential risk factor for adverse birth outcomes.