A systematic review and meta-analysis investigated whether metformin use by mothers with gestational diabetes mellitus (GDM) increases the risk of small-for-gestational-age (SGA) infants. It included 19 controlled trials with 1921 participants, searching PubMed, Embase and the Cochrane Library until 1 September 2024. Maternal metformin use was not significantly associated with the incidence of SGA (OR = 1.10, 95% CI: 0.97–1.24, p = 0.14). There were no significant differences versus insulin (n = 27,622, OR = 1.25, 95% CI: 0.91–1.73, p = 0.17) or placebo (n = 1,685, OR = 1.36, 95% CI: 0.80–2.32). Two studies (n = 554) showed a lower incidence of SGA with metformin versus diet modification (OR = 0.50, 95% CI: 0.29–0.87, p = 0.01). More research is needed on the long-term effects of metformin on offspring. Registration: CRD420251141984.