Background: Gestational diabetes mellitus (GDM) and excessive gestational weight gain have significant implications for the health of both mother and child. Our objective was to detail gestational weight gain in women in relationship to GDM. Methods: Data were collected by retrospective reviews of medical records in women who delivered between January and December 2007 at the Laval University Medical Center (Quebec, Canada). The analysis included 294 women (55 GDM and 239 controls) for whom gestational weight gain was calculated by the difference between maternal weight measured at delivery, or at the last prenatal visit (≥37th week), and prepregnancy self-reported weight. Gestational weight gain and rate of weight gain were also calculated for each trimester and until GDM screening. Gestational weight gain was compared to the 2009 recommendations by the Institute of Medicine (IOM). Women with GDM were diagnosed and treated according to the Canadian Diabetes Association guidelines. Results: Weight gain in the first trimester was significantly higher in GDM patients compared to controls (3.40 ± 0.42 vs. 1.87 ± 0.16 kg, p ≤ 0.01) and was above IOM recommendations, whereas weight gain in the third trimester was significantly lower in GDM patients compared to controls (4.11 ± 0.36 vs. 6.35 ± 0.18 kg, p ≤ 0.0001). Prepregnancy body mass index (BMI) and first trimester weight gain were both significant and independent predictors of GDM (odds ratio [OR] 1.11, 95% confidence interval [CI] 1.05-1.17, and OR 1.25, 95% CI 1.10-1.42, respectively). Conclusions: First trimester gestational weight gain may need more clinical attention, as it has been identified as an independent and significant risk factor for GDM independent of traditional risk factors, including preconception obesity.