BACKGROUND: The aim of this study was to investigate the relationship between maternal body mass index (BMI), a modifiable factor during the reproductive period, and inflammation and oxidative stress by assessing dynamic thiol-disulfide homeostasis (TDH) in both the mother and fetus. METHOD: This prospective cohort study was conducted between May and June 2024 at a tertiary obstetric care center. The inclusion criteria consisted of healthy pregnant women aged over 18 years, between 37 and 41 weeks of gestation, who had not used medications other than iron and folic acid supplements, with newborns birth weight between 2,500 grams (g) and 4,500 g, and Apgar scores ≥ 7 at the 5th minute after birth. Maternal peripheral blood (5 mL) was collected at delivery admission, and 3 mL of fetal blood was obtained from the umbilical cord after delivery. Participants (n = 125) were categorized into three BMI-based groups: (1) non-obese at both pre-pregnancy and delivery (BMI <
30 kg/m², n = 72)
(2) non-obese at pre-pregnancy but gained weight to a BMI classified as obese at delivery (BMI <
30 kg/m² pre-pregnancy, ≥ 30 kg/m² at delivery, n = 29)
and (3) obese at both pre-pregnancy and delivery (BMI ≥ 30 kg/m², n = 24). RESULTS: Maternal serum native thiol (SH) (306.21 ± 49.19 µmol/L vs. 270.9 ± 60.12 µmol/L vs. 276.9 ± 59.18 µmol/L, p = 0.004) and total thiol (SH + SS) (337.88 ± 52.43 µmol/L vs. 303.8 ± 62.13 µmol/L vs. 306 ± 58.01 µmol/L, p = 0.006) levels were significantly higher in the non-obese at both pre-pregnancy and delivery group compared to the other groups. Disulfide (SS) levels and thiol-disulfide ratios (SS/SH, SS/total thiol, and SH/total thiol) showed no significant differences among groups (p >
0.05, for all). In fetal cord blood, SH, SS, SH + SS levels, and thiol-disulfide ratios were not significantly different among the groups (p >
0.05, for all). CONCLUSION: Maternal obesity, whether longstanding or newly developed during pregnancy, disrupts TDH and reduces antioxidant capacity, increasing susceptibility to oxidative damage and may affect maternal and fetal health. CLINICAL TRIAL NUMBER: Not applicable.