Background
Prevention of excessive gestational weight gain during pregnancy is difficult; targeting women before pregnancy may be more effective.

Aims
In order to generate knowledge that may influence the development of effective interventions to promote healthy weight in reproductive‐aged women, this study aimed to explore knowledge and belief formation regarding gestational weight gain for preconception and pregnant women.

Materials and Methods
Women ≥18 years (preconception n = 265; pregnant women at 16 weeks gestation n = 271) completed questionnaires assessing knowledge and beliefs about gestational weight gain. Responses were categorised according to the 2009 Institute of Medicine gestational weight gain recommendations.

Results
Preconception women exhibited poorer gestational weight gain knowledge than pregnant women, yet only half of pregnant women reported accurate gestational weight gain knowledge within the Institute of Medicine recommendations. Beliefs about gestational weight gain were also inaccurate for both preconception and pregnant women, with 34.1% of pregnant and 44.6% of preconception women expecting to gain less than recommendations. Gestational weight gain knowledge accounted for about half of the variance in gestational weight gain beliefs.

Conclusions
Overall, the large inaccuracies in gestational weight gain knowledge and beliefs reported by both preconception and pregnant women suggest significant gaps in dissemination of gestational weight gain advice throughout the reproductive life phase. Knowledge is an important part of belief formation that can lead to appropriate weight gain. Hence, health professionals and policy makers should actively pursue opportunities to improve gestational weight gain knowledge in reproductive‐aged women.