Plasma C-peptide reflects the insulin-secretory activity of pancreatic β-cells which modulates fetal growth. Cord blood C-peptide levels were measured in women with gestational diabetes mellitus (GDM) and in women with normal glucose tolerance (NGT). Forty-one women underwent a 75-g oral glucose tolerance test (18 GDM, 23 NGT). Cord blood C-peptide (p = 0.09) and glucose levels (p = 0.08) from newborns of GDM women tended to be higher than those from NGT women. In the entire group, cord blood C-peptide correlated with maternal insulin, fasting C-peptide, insulin sensitivity, interleukin-6, weight and body mass index measured at screening (ρ from 0.34 to 0.48, all p < 0.05) and tended to correlate with offspring weight (ρ = 0.28, p = 0.08). Newborns of GDM women tended to have elevated cord blood C-peptide which correlated with maternal insulin, insulin sensitivity and anthropometric measures at diagnosis and with offspring characteristics. This suggests that insulin-secretory activity of the newborn is related to maternal metabolic parameters.