Abstract
BACKGROUND AND AIMS: Infertility affects approximately 18.3% of couples in Iran, and in vitro fertilization (IVF) remains a widely used treatment. Vitamin D deficiency and elevated body mass index (BMI) have been linked to poorer reproductive outcomes in some populations, but region-specific evidence among Iranian women is limited. This study aimed to investigate the association between serum 25-hydroxyvitamin D [25(OH)D] levels, BMI, and IVF outcomes in Iranian women. METHODS: This case-control study was conducted at Fatemieh Medical Training Center, Hamadan, Iran. A total of 314 women undergoing IVF were included (157 with successful clinical pregnancy confirmed by positive β-hCG test and 157 with unsuccessful outcomes). Serum 25(OH)D levels were measured using enzyme-linked immunosorbent assay (ELISA). BMI was calculated as weight (kg)/height (m²) and categorized as normal (18.5-24.9), overweight (25-29.9), and obese (≥ 30). Exclusion criteria included systemic diseases, insulin resistance, polycystic ovary syndrome (PCOS), and diabetes. Data were collected using a researcher-developed checklist from patient records. Statistical analyses were performed using Stata version 14. Pearson correlation coefficients were used to assess associations between continuous variables, and multivariable logistic regression was applied to estimate odds ratios (ORs) with 95% confidence intervals (CIs) for IVF failure, adjusting for age. All tests were two-sided with a significance level of p < 0.05. RESULTS: Mean serum 25(OH)D levels were significantly lower in the unsuccessful IVF group (17.8 ± 5.9 ng/mL) compared to the successful group (24.6 ± 6.8 ng/mL; mean difference = 6.8 ng/mL, p < 0.001). BMI showed a moderate negative correlation with 25(OH)D levels (r = -0.38, p < 0.01). Obese women (BMI ≥ 30) had a higher likelihood of IVF failure (adjusted OR = 2.3, 95% CI: 1.4-3.7, p = 0.001). Participant age (mean 33.2 ± 5.1 years, range 20-45) showed a weak positive correlation with unsuccessful outcomes (r = 0.15, p = 0.04). CONCLUSION: Vitamin D deficiency and higher BMI are independently associated with increased risk of IVF failure among Iranian women. Optimizing vitamin D status and achieving a healthy BMI prior to IVF treatment may improve success rates. Given the observed association, routine assessment and supplementation of vitamin D in women of reproductive age preparing for assisted reproductive technologies should be considered. Future studies should adjust for additional potential confounders, including chronic inflammation and lifestyle factors.