Whether the placenta contributes to some of the abnormal hormonal profiles in gestational diabetes mellitus (GDM) pregnancies and whether GDM affects placental endocrine signaling pathways are yet to be established. The objective of this study was to investigate differences in the expression of the placental steroid and peptide hormone synthesis-related factors, enzymes and their receptors between normal and GDM pregnancies. Nine term placentae from GDM pregnancies and twelve from healthy pregnancies were collected. The results of immunohistochemistry, Western blotting and semiquantitative RT-PCR indicated that mRNA and protein levels of leptin, leptin receptors, androgen receptor and FGF2 were significantly higher in the GDM placentae than non-GDM placentae; while NRIH3, NRIH2, StARD3, CYP11A1, HSD3B, HSD11B, HSD17B, ERalpha, ERbeta, progesterone receptor, FGF receptor-2, insulin receptor-alpha and -beta showed no differences. Interestingly, Western blotting and immunohistochemistry revealed that aromatase protein concentrations in the GDM placentae were significantly reduced without a change in mRNA levels. Moreover, androgen upregulated FGF2 expression in the placental villous explants. These findings suggest that the placentae of GDM pregnancies contribute to elevated testosterone and leptin levels due to a decrease in the conversion of testosterone to estrogens and to an increase in leptin production. The androgen and leptin signaling pathways may be over-activated by the presence of excessive ligands and overexpressed receptors in GDM placentae. Dysregulation of these two endocrine networks may contribute to placental abnormalities eventually increasing the frequency of maternal and fetal complications associated with GDM.