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PURPOSE This was a prospective observational cohort study that aimed to determine whether fetal sex influences the maternal and fetal outcomes of gestational diabetes mellitus (GDM). METHODS In this study, 327 European primiparous women were consecutively recruited after diagnosis of GDM. AUC on the oral glucose tolerance test (OGTT), need for insulin therapy, maternal and obstetrical outcomes, and fetal fat mass (by measuring the thickness of the anterior abdominal subcutaneous tissue) were recorded and compared between the two subgroups of female and male fetuses. FINDINGS Despite the absence of differences in multiple comparisons of the OGTT, the AUC-OGTT was significantly higher in women carrying a male fetus (22.6 [3.2] mmol/L vs 19.7 [2.8] mmol/L). The abdominal fat thickness appeared to increase with gestational age, with higher growth in male fetuses than in female fetuses. The overall risk of need for insulin therapy was significantly higher in women carrying a male fetus (odds ratio = 1.837). At delivery, birthweight was higher in males than in females only if adjusted for gestational age, similarly for placental weight, otherwise there were no significant differences between the groups in total length of gestation, rates of cesarean delivery, and Apgar scores. IMPLICATIONS Overall, our data propose an association between fetal sex and GDM outcomes, suggesting the hypothesis that in maternal-fetal interactions, the fetus can affect maternal glucose metabolism.