This randomised controlled trial verified the efficacy of an implementation intentions intervention (n = 24) to promote fruit and vegetable consumption among childbearing age women at risk for gestational diabetes mellitus against a control condition based on the question–behaviour effect (n = 26). There was only a significant time effect (F(2,85) = 5.69, p = 0.0048). Both groups increased their vegetable consumption compared to baseline at 3 months (p = 0.0022) and 6 months (p = 0.0042). There were no significant effects on weight, waist circumference and blood glucose levels. Implementation intentions and the question–behaviour effect appear to be effective behaviour change techniques to promote vegetable intake among this high-risk population.