Smoking during pregnancy and gestational diabetes mellitus: a systematic review and meta-analysis

PURPOSE: To investigate whether maternal cigarette smoking during pregnancy is a risk factor for developing GDM. METHODS: MEDLINE, Scopus, CENTRAL and Google Scholar databases were searched from inception to December 2022 to identify eligible original articles. A systematic review and meta-analysis (weighted data, random-effects model) were performed. The primary outcome was the development of GDM in pregnant women. The results were expressed as odds ratios (OR) with 95% confidence interval (CI) (inverse variance method). Subgroup analysis was planned according to the maternal smoking status and GDM diagnostic criteria. Statistical heterogeneity was checked with the Chi-squared (Chi(2)) test and the I(2) index was used to quantify it. The studies were evaluated for publication bias. RESULTS: Thirty-five studies, including 23,849,696 pregnant women, met the inclusion criteria. The pooled OR of smoking during pregnancy compared with non-smoking (never smokers and former smokers) was 1.06 (95% CI 0.95–1.19), p = 0.30; I(2) = 90%; Chi(2) = 344; df=34; p < 0.001. Subgroup analysis was performed according to the two-step Carpenter-Coustan diagnostic criteria, due to the high heterogeneity among the other applied methods. The pooled OR for the Carpenter-Coustan subgroup was 1.19 (95% CI 0.95–1.49), p = 0.12; I(2) = 63%; Chi(2) = 27; df=10; p < 0.002. Further subgroup analysis according to maternal smoking status was not performed due to missing data. CONCLUSION: There is no evidence to support an association between maternal cigarette smoking during pregnancy and the risk for GDM. Universally accepted diagnostic criteria for GDM must be adopted to reduce heterogeneity and clarify the association between smoking and GDM.