Excessive pregnancy weight gain is associated with adverse maternal and child health outcomes. Intervention developers have assumed that adopting a healthier diet and increasing physical activity in pregnancy can limit weight gain, but evaluations of such interventions have yielded mixed results. Recent reviews of this literature have not identified defining characteristics of effective interventions. We systematically reviewed 10 published controlled trials of interventions that aimed to reduce gestational weight gain through changes in diet or physical activity. Characteristics of the sample, intervention content and delivery, and methodology were categorized. Meta-analysis showed that, overall, diet and physical activity change was effective in reducing gestational weight gain, but there was considerable heterogeneity in outcomes. Our analysis points to sample characteristics and aspects of intervention design, content, delivery and evaluation which differ between studies and may explain variation in effectiveness. Failure to evaluate changes in behaviour or its psychological determinants, and under-reporting of intervention content, may obscure identification of the processes by which weight change is effected. This limits our ability to discern active intervention ingredients. We suggest that behaviour-based gestational weight gain reduction interventions be more systematically designed, evaluated and reported to build on insights from behavioural science.