BACKGROUND: Research documents social and economic antecedents of adverse birth outcomes, which may include involuntary job loss. Previous work on job loss and adverse birth outcomes, however, lacks high-quality individual data on, and variation in, plausibly exogenous job loss during pregnancy and therefore cannot rule out strong confounding.

METHODS: We analysed unique linked registries in Denmark, from 1980 to 2017, to examine whether a father's involuntary job loss during his spouse's pregnancy increases the risk of a low-weight (i.e. <2500 grams) and/or preterm (i.e. <37 weeks of gestational age) birth. We applied a matched-sibling design to 743 574 sibling pairs.

RESULTS: Results indicate an increased risk of a low-weight birth among infants exposed in utero to fathers' unexpected job loss [odds ratio (OR) = 1.37, 95% confidence interval (CI): 1.07, 1.75]. Sex-specific analyses show that this result holds for males (OR = 1.70, 95% CI: 1.14, 2.53) but not females (OR = 1.24, 95% CI: 0.80, 1.91). We find no relation with preterm birth.

CONCLUSIONS: Findings support the inference that a father's unexpected job loss adversely affects the course of pregnancy, especially among males exposed in utero.