Nineteen trials (1,998 participants) were included in the review. Trials generally scored well on individual quality items (except for blinding), but the overall quality of the evidence base was rated low or very low.
Metformin versus no metformin (three trials): There were no statistically significant differences for the risk of gestational diabetes mellitus (I2=59%) or macrosomial infant (I2=18%).
Low glycaemic index diet versus high glycaemic/low fat diet advice (three trials): Low glycaemic index diet was more effective in reducing the risk of large for gestational age infant (RR 0.14, 95% CI 0.05 to 0.41; I2=43%). No statistically significant difference was found for maternal fasting glucose (I2=40%).
Any dietary counselling versus usual care (eight trials): Dietary counselling was more effective in reducing the risk of gestational diabetes mellitus (RD -0.05, 95% CI -0.10 to -0.01; seven trials, I2=41%). There were no statistically significant differences in maternal fasting glucose (three trials; I2=0%), or risk of macrosomial infant (five trials; I2=77%).
Exercise versus usual care (three trials): Exercise training was more effective in reducing the risk of macrosomial infant (RR 0.36, 95% CI 0.13 to 0.99; two trials, I2=38%). No statistically significant differences were found for maternal fasting glucose (two trials; I2=0%) or risk of gestational diabetes mellitus (three trials; I2=66%).
A statistically significant reduction in incidence of gestational diabetes mellitus was found with probiotics with dietary counselling versus dietary counselling alone (one trial). No statistically significant differences were found for the incidence of gestational diabetes mellitus or macrosomia from self-monitoring weight gain versus standard care (one trial).
The removal of trials in the sensitivity analyses showed significant effects on maternal fasting blood glucose following dietary counselling (WMD -0.29mmol/L, 95% CI -0.55 to -0.03), and advice on low glycaemic diet (WMD -0.27mmol/L, 95% CI -0.51 to -0.03). Further exploration of heterogeneity was reported in the paper.