Timing of smoking cessation before and during pregnancy and risk of gestational diabetes: A systematic review.

Introduction: Gestational diabetes mellitus (GDM) is one of the most common metabolic complications of pregnancy, with a steadily increasing global prevalence. Cigarette smoking is a well-established modifiable risk factor for metabolic disorders; however, its association with gestational diabetes mellitus remains unclear, partly due to the widespread use of simplified, binary exposure definitions. Of particular interest is the timing of smoking cessation before or during pregnancy, as it may differentially influence the risk of glucose dysregulation during gestation. Objective: The objective of this systematic review was to synthesize available evidence from original studies that classify maternal smoking according to cessation timing-before pregnancy, during early pregnancy, or continued smoking beyond the first trimester-and to examine the association of these exposure patterns with gestational diabetes mellitus and pregnancy-related dysglycemia. Methods: This review was designed and reported in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. A comprehensive systematic search was conducted in MEDLINE, Embase, Scopus, and Web of Science from inception with no date restrictions. Eligible studies were original observational investigations including pregnant women without pregestational diabetes, in which smoking exposure was clearly defined with respect to cessation timing. The primary outcome was gestational diabetes mellitus, while secondary outcomes included pregnancy-related dysglycemia. Due to substantial heterogeneity in exposure definitions and outcome assessment, results were synthesized narratively. Results: Of 1672 identified records, four studies met the inclusion criteria. Overall, continued smoking beyond early pregnancy was consistently associated with an increased risk of gestational diabetes mellitus. In contrast, smoking cessation before pregnancy or during the first trimester appeared to attenuate or eliminate this excess risk, although some estimates did not reach statistical significance due to limited statistical power. Findings related to pregnancy dysglycemia further highlighted the complex interplay between smoking behavior, gestational weight gain, and maternal glucose metabolism. Conclusions: The timing of smoking cessation is a critical modifier of gestational diabetes mellitus risk. Continued smoking after the first trimester is associated with increased metabolic risk, whereas early cessation before or during early pregnancy appears beneficial. These findings underscore the importance of timing-sensitive smoking exposure assessment in both epidemiological research and clinical counseling of pregnant women. However, these findings are based on a very limited evidence base, as only four observational studies met the inclusion criteria, substantially restricting the strength and generalizability of inferences.