Objectives Little is known about the antecedents to dietary and physical activity behaviours that can support healthy gestational weight gain (GWG) across different weight status groups in pregnancy. The aim of this study was to use constructs common to dominant health behaviour theories to determine if predisposing, reinforcing and enabling factors for healthy eating, physical activity and weight gain differed between healthy and overweight pregnant women. Methods Pregnant women (n = 664) aged 29 ± 5 (mean ± SD) years were recruited at 16 ± 2 weeks gestation. Measures were self-reported pre-pregnancy weight, psychosocial constructs for healthy eating, physical activity and GWG and demographic data. Height was measured at 16 weeks. Psychosocial constructs were compared between women with pre-pregnancy weight status of healthy (BMI < 25 kg/m2) and overweight (BMI ≥ 25 kg/m2). Results Health behaviour intentions, positive outcome expectations and social support for healthy eating and physical activity were not different between healthy (66 %) and overweight (34 %) women. Overweight women had lower self-efficacy for healthy eating, physical activity and GWG (p < 0.001), higher negative outcome expectations for GWG (p = 0.004), and higher barriers to healthy eating (p = 0.002), and physical activity (p = 0.006). Conclusions for practice Both healthy and overweight women appear motivated to follow a healthy diet, exercise and avoid excess gestational weight during pregnancy. However many psychosocial factors associated with achieving these goals were different between healthy and overweight women. Health behaviour interventions tailored to overweight pregnant women should consider improving self-efficacy, providing support to overcome perceived barriers, validate positive changes made, and assist in managing negative expectations.