To examine the independent and joint associations of maternal smoking during pregnancy and gestational diabetes mellitus (GDM) on the risk of preterm birth and newborn perinatal complications in a large national cohort of primiparous women, with an area of limited existing research.

This register-based cohort study included 318 783 singleton births among primiparous women in Finland (2004–2018), of which 290 602 were full-term. Data on GDM, smoking status, maternal characteristics, pregnancy duration, newborn hospitalization, and mortality were obtained from the Finnish Medical Birth Register. Associations were examined using linear, logistic, and Poisson regression. Interaction between GDM and maternal smoking was assessed on multiplicative and additive scales.

GDM and continued smoking were independently associated with shorter pregnancy duration and an increased risk of newborn hospitalization beyond 1 week. Continued smoking also increased the risks of preterm birth and perinatal mortality. On the additive scale, combined exposure to GDM and maternal continued smoking produced higher absolute risks than expected from either exposure alone. Additive interaction was observed for preterm birth (RERI = 0.44, 95% CI: 0.16–0.67), gestational duration (RERI = 2.05 days, 95% CI: 1.07–3.04), and delayed hospital discharge (RERI = 0.36, 95% CI: 0.27–0.39). On the multiplicative scale, interaction was detected only for gestational duration (p = 0.008).

Both GDM and maternal smoking contribute independently to adverse perinatal outcomes, but their combined exposure elevates absolute risks more than either factor alone, particularly for reduced gestational length, preterm birth, and delayed newborn discharge. These findings highlight the importance of addressing both smoking and GDM in prenatal care, even among generally low-risk primiparous women.