Objective: Poor sleep quality is common during pregnancy. Although a few studies have investigated the associations between maternal sleep quality and fetal birth weight (BW), no evidence has been clearly demonstrated. Our aim was to investigate the effects of sleep quality during pregnancy on the newborn BW z-scores. Participants: 1466 mother-infant pairs were included in the present study based on an ongoing prospective cohort. Methods: Questionnaires including the Pittsburgh Sleep Quality Index (PSQI) and scales for psychosocial status were administered at each trimester. BW z-scores were calculated based on the INTERGROWTH-21st standard. A generalized estimating equation model was applied to evaluate the associations between trimester-specific sleep quality and newborn BW after adjusting for potential confounders. Multivariable logistic regression models were applied to examine the impacts of maternal sleep quality on small-for-gestational-age (SGA) or low birth weight (LBW). Results: We found that maternal PSQI scores in the first and third trimesters were negatively associated with BW z-scores among female newborns (β = -0.032, 95% CI: -0.063, -0.001, P= .043; β = -0.031, 95% CI: -0.060, -0.003, P= .033, respectively). However, no relationship was observed between maternal sleep quality and BW in male neonates. Additionally, poor sleep quality in late pregnancy was a risk factor for LBW (OR = 1.501, 95% CI: 1.082-2.082). Conclusions: The BW z-scores of female newborns decreased as maternal sleep quality in the first and third trimesters worsened. This finding suggests that sleep during pregnancy may influence fetal weight in a trimester- and gender-specific manner.