Abstract
OBJECTIVE: This study aimed to investigate the association between smoking behaviours during early pregnancy and the risk and severity of gestational diabetes mellitus (GDM), with a particular focus on smoking status, smoking intensity and secondhand smoke exposure. DESIGN: Secondary analysis of prospectively collected cohort data. SETTING: Multi-centre study conducted in South Korea (Korean Pregnancy Outcome Study) between March 2013 and January 2017. PARTICIPANTS: From 4537 pregnant women initially enrolled, 3457 singleton pregnancies were included after excluding cases with transfer, loss to follow-up, twin pregnancies, miscarriages and pre-existing diabetes mellitus. All participants were women of Korean ethnicity. SECONDARY OUTCOME MEASURES: Primary outcome was GDM and its subtypes (A1GDM: diet-controlled; A2GDM: insulin-requiring). Secondary outcomes were associations with active smoking (before pregnancy and during early pregnancy), smoking intensity dose-response relationships (pack-years) and secondhand smoke exposure among never-smokers. RESULTS: Among 3457 participants, 231 women (6.7%) were diagnosed with GDM (198 A1GDM, 33 A2GDM). Active smoking before pregnancy (adjusted OR (aOR) 3.98, 95% CI 1.58 to 9.30) and during early pregnancy (aOR 9.90, 95% CI 2.97 to 29.45) were significantly associated with A2GDM, while no significant association was observed with A1GDM. A clear dose-response relationship was observed, with smoking intensity >4 pack-years markedly increasing A2GDM risk (aOR 20.68, 95% CI 6.75 to 59.39). Detailed pack-year analysis showed 4-6 pack-years (aOR 20.57, 95% CI 5.80 to 65.46) and >6 pack-years (aOR 25.98, 95% CI 3.21 to 146.45). Among never-smokers, secondhand smoke exposure showed a borderline association with overall GDM risk (aOR 1.33, 95% CI 0.98 to 1.81). CONCLUSIONS: Maternal active smoking before and during early pregnancy, as well as higher smoking intensity, was associated with an increased risk of pharmacologically treated GDM (A2GDM). Although secondhand smoke exposure did not reach statistical significance, the trend suggested a potential association with GDM risk among never-smokers. These findings provide important evidence for public health strategies for prenatal care, as smoking cessation and environmental smoke avoidance during prenatal and early antenatal care in women reduce the risk of gestational diabetes.