Introduction Shift work is associated with higher risk for mothers during pregnancy and increases future obesity risk for both mothers and children. However, only limited literature examined the effect of shift work on maternal body weight changes during pregnancy and after delivery.

Objectives To examine the association between maternal shift work and body weight changes during pregnancy periods, and to identify possible mediating factors.

Methods We extracted maternal weight and body mass index (BMI) during pregnancy and maternal shift work status from a population-based cohort study. The associations between maternal shift work and maternal weight retention at six months were evaluated with multivariable linear models, and we examined the mediation and interaction effect from excess gestational weight gain.

Results We included 13 575 mothers giving birth to term singleton in the study population, and maternal shift work before pregnancy was associated with a 0.25-kilogram higher postpartum weight retention at six months after adjusting for confounders (95% CI: 0.11–0.40, p=0.001). Meanwhile, this association was mediated by excess gestational weight gain (Natural indirect effect: 0.12-kilogram increase, 95% CI: 0.07–0.17, p-value < 0 .001) and nonemployment during pregnancy (Natural indirect effect: 0.02-kilogram increase, 95% CI: 0.00–0.03, p-value 0.006). Lastly, a 0.39-kilogram additive interaction (95% CI: 0.12–0.65, p-value 0.005) was identified for excessive gestational weight gain on the association between maternal shift work and postpartum weight gain at six months.

Conclusion Maternal shift work before pregnancy is associated with increased postpartum weight retention at six months, and this association was mediated by nonemployment status during pregnancy. Meanwhile, excessive gestational weight gain exacerbates the effect through additive interaction and mediation.