Introduction: Maternal smoking during pregnancy and gestational diabetes mellitus (GDM) have opposite effects on fetal growth during pregnancy. The aim of the study was to evaluate the interaction of smoking during pregnancy and gestational diabetes mellitus on head circumference and birthweight of newborns.

Material and methods: The study included all primiparous women with singleton pregnancies (n = 290 602) without previously diagnosed diabetes or hypertension in Finland between 2006 and 2018. The information on gestational diabetes mellitus, newborn birthweight and head circumference, and maternal smoking and backgrounds was derived from the Finnish Medical Birth Register. Linear regression models were used in the analyses.

Results: In total 8.0% of parturients quit smoking during the first trimester and 9.9% continued smoking thereafter. The prevalence of GDM was 8.9% (n = 25 948). Newborns of women who continued smoking had a smaller head circumference (b = -0.24, SE = 0.01, p < 0.0001) and birthweight (b = -0.28, SE = 0.01, p < 0.0001) compared to newborns of women who did not smoke. Head circumference and birthweight were greater in newborns of women with GDM (b = 0.09, SE = 0.01, p < 0.0001 and b = 0.16, SE = 0.01, p < 0.0001, respectively) compared to newborns of women without GDM. In the interaction analyses, head circumference (b = -0.13, SE = 0.01, p < 0.0001) was smaller and birthweight (b = -0.13, SE = 0.02, p < 0.0001) was lower in newborns of women with GDM who continued smoking compared to newborns of women without GDM who did not smoke.

Conclusions: Although smoking and GDM have opposite effects on fetal growth, the negative effects of exposure to smoking are also seen in newborns of women with GDM. Compared to smoking after the first trimester of pregnancy, cessation of smoking during the first trimester was associated with greater head circumference and birthweight in newborns.