The prevalence of gestational diabetes mellitus (GDM) and obesity is increasing in developed countries, presenting significant challenges to acute care and public health. The aim of this study is to systematically review published controlled trials evaluating behavior modification interventions to prevent the development of GDM. Nine studies were identified involving such techniques as repetition of information, use of verbal and written educational information, goal setting, and planning, in addition to group and individual counseling sessions. Of the 3 trials with GDM incidence as a primary outcome, only 1 showed a significant reduction. GDM was a secondary outcome in 6 studies where the prevention of excessive gestational weight gain was the primary outcome and only 1 trial study determined an effective intervention. The small number of effective interventions highlights a significant gap in evidence to inform maternity health policy and practice.