Both pre-pregnancy body mass index (BMI) and gestational weight gain (GWG) are important determinants of a healthy pregnancy outcome and may show important variation. To study the influence of socio-demographic and obstetrical correlates on pre-pregnancy BMI and GWG, data of 54 022 singleton term pregnancies were analysed using adjusted regression models. In 2009, in the Northern region of Belgium, one-third of women were overweight (21.6%) or obese (10.1%) and GWG as recommended by the Institute of Medicine occurred in only 28% of obese women. A high pre-pregnancy BMI was significantly associated with low maternal education, high maternal age and multiparity, belonging to ethnic minority groups and a lower professional state. Compared to adequate GWG, excessive GWG was more common in younger (<20 years) women, with higher pre-pregnancy BMI and pregnancy-induced hypertension. Moreover, younger (20–24 years), single women, belonging to ethnic minority groups showed higher odds for excessive as well as insufficient GWG, while those with high/highest educational level had lower odds for excessive (odds ratio [OR] 0.76; confidence interval [CI] 0.72–0.80) and insufficient (OR 0.93; CI 0.89–0.98) GWG. The results of this study highlight the scale of the problem of maternal obesity and excessive GWG for this region and offer opportunities to target educational campaigns and intervention programmes in the clinical setting.