**Background:** Gestational diabetes mellitus (GDM) is a common pregnancy complication with rising prevalence worldwide, including in China where incidence reached 17.42% in 2018–2019. Folic acid (FA) supplementation is universally recommended before and during pregnancy to prevent neural tube defects, but its relationship with GDM risk remains controversial. Pre-pregnancy body mass index (BMI) is a well-established risk factor for GDM, and obesity may affect folate metabolism and serum folate levels. However, the joint effect of FA supplementation and pre-pregnancy BMI on GDM risk had not been fully addressed. This study aimed to examine the interaction between FA supplements and pre-pregnancy BMI on GDM risk, including the timing of supplementation.
**Methods:** This prospective cohort study recruited 3,186 pregnant women at 4–14 weeks of gestation from the Qingdao Women and Children's Hospital Health Cohort between May 1, 2019, and June 27, 2021. After exclusions (multiple pregnancy, termination/abortion, loss to follow-up, missing OGTT data, incomplete FA or BMI information, and pre-existing diabetes), 2,095 women were included. FA supplement use was assessed at enrollment, including brand, daily dose, and duration. Sufficient FA (FA-S) was defined as ≥ 400 µg/day; deficient FA (FA-D) as < 400 µg/day. Pre-pregnancy BMI was categorized as underweight/normal (< 25.0 kg/m²), overweight (25.0–29.9 kg/m²), and obese (≥ 30.0 kg/m²). GDM was diagnosed at 24–28 weeks using a 75 g oral glucose tolerance test (OGTT) with thresholds: fasting ≥ 5.1 mmol/L, 1-hour ≥ 10.0 mmol/L, or 2-hour ≥ 8.5 mmol/L. Logistic regression was used to calculate odds ratios (OR) and adjusted odds ratios (aOR) with 95% confidence intervals (CI). Interaction on the multiplicative scale was assessed using product terms and ratio of odds ratios (ROR). Covariates included age, education, income, smoking, drinking, family history of diabetes, fertilization method, history of GDM, and vitamin B12 supplement use.
**Key Results:** The incidence of GDM was 17.76% (372/2,095). Overall, 8.88% (186/2,095) of women had FA-D (< 400 µg/day). Compared with normal-weight women (BMI < 25.0 kg/m²), overweight women had significantly higher GDM risk (aOR = 2.28, 95% CI: 1.71–3.03, P < 0.001) and obese women had even higher risk (aOR = 3.91, 95% CI: 2.27–6.74, P < 0.001). FA supplement dose alone was not associated with GDM (FA ≥ 400 vs. < 400 µg/day: aOR = 1.49, 95% CI: 0.53–4.23, P = 0.453). In the joint analysis using the reference group of normal-weight women with FA-S, the aORs were: overweight + FA-S = 2.17 (95% CI: 1.60–2.95), overweight + FA-D = 3.27 (95% CI: 1.55–6.92), obese + FA-S = 3.57 (95% CI: 2.02–6.34), and obese + FA-D = 10.82 (95% CI: 1.69–69.45). However, the ROR comparing FA-D vs. FA-S within obese women was 2.70 (95% CI: 0.47–2.30) and within overweight women was 0.66 (95% CI: 0.30–1.49), indicating no statistically significant difference in GDM risk between FA supplement dose groups within each BMI category. After stratifying by FA supplementation time (< 3 months vs. ≥ 3 months), the interaction remained non-significant (ROR = 1.63, 95% CI: 0.37–7.04 for obese women with < 3 months of FA). Food folate intake and total folate intake were also not associated with GDM risk (P-trend = 0.980 and 0.919, respectively).
**Clinical Implications:** This study provides strong evidence that pre-pregnancy BMI is a dominant risk factor for GDM, while FA supplementation dose from pre-pregnancy to early pregnancy does not significantly modify this risk. Even with adequate FA intake (≥ 400 µg/day), obese women had substantially elevated GDM risk (aOR = 3.57). The findings suggest that clinical strategies for GDM prevention should prioritize weight management and BMI optimization before pregnancy rather than adjusting FA supplementation doses. The lack of interaction between FA dose and BMI indicates that current FA supplementation recommendations (400 µg/day for NTD prevention) can be maintained regardless of BMI without concern for increasing GDM risk. However, the study had limitations including small sample size in the FA-D/obese subgroup (n = 5), reliance on self-reported FA intake rather than serum folate levels, and lack of analysis of FA intake throughout the entire pregnancy. Larger studies with biochemical folate measurements are needed to further clarify these relationships.