Background
Health behaviour theories acknowledge that beliefs, attitudes and knowledge contribute to health behaviours, yet the role of these cognitions in predicting weight gain during pregnancy has not been widely researched.

Aims
To explore and compare the predictive nature of gestational weight gain (GWG) expectations and knowledge on weight gain during pregnancy.

Materials and Methods
One hundred and sixty‐six women were tracked during pregnancy. Participants provided information on prepregnancy weight, height, GWG expectations and knowledge at 16–18 weeks’ gestation (Time 1). To calculate gestational weight gain, prepregnancy weight was subtracted from weight at 36 weeks’ gestation (collected at Time 2). Gestational weight gain above the Institute of Medicine’s GWG recommendations was classified as excessive. A hierarchical regression examined the predictive nature of GWG expectations for actual GWG. Chi‐square significance tests determined whether the accuracy of GWG knowledge differed depending on GWG status and prepregnancy BMI category.

Results
GWG expectations were a significant predictor of weight gain during pregnancy. Women who experienced excessive GWG were more likely to overestimate the minimum amount of weight that they needed to gain to have a healthy baby.

Conclusions
GWG expectations are predictive of actual GWG, and GWG knowledge among women is generally poor. In particular, overestimating of the minimum amount of weight to gain during pregnancy is associated with excessive GWG. As such, it may be beneficial to design interventions to prevent excessive GWG that targets these cognitions.