When compared with the insulin group, the pooled result showed no significant difference between the groups in LGA or SGA rates (large for age and small for age respectively) or hypoglycemia rates. But the incidence of preterm birth was significantly higher in metformin group (3 studies, n = 1110, P = 0.01, OR = 1.74, 95%CI [1.13 to 2.68]). And the average gestational ages at delivery were significantly lower in the metformin group (5 trials, n = 1270, P = 0.02, SMD = −0.14, 95%CI [−0.25 to −0.03]).

Conclusions

The researchers concluded that metformin is comparable to insulin in glycemic control, which it achieves by suppressing hepatic gluconeogenesis, increasing insulin sensitivity, and enhancing peripheral glucose uptake. 

They noted that these effects are useful during pregnancy, when glucose control deteriorates as a result of changes in insulin resistance. They stated, “Metformin could be used in women with GDM in view of the comparative glycemic control and neonatal outcomes, especially for those with mild GDM.” 

However, they warned, “the risks of preterm birth could not be ignored.” They advised clinicians to weigh these factors, based on the condition of each patient, and they recommended further studies with larger sample size for a more thorough analysis.

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