Vitamin D and Infertility: A Case-Control Study Conducted in Baghdad City

Background: Vitamin D plays an important role in reproductive physiology through its effects on endocrine, metabolic, and immunological pathways. Despite abundant sunlight, vitamin D deficiency is highly prevalent among women of reproductive age in the Middle East and may contribute to infertility. Objectives: To assess serum vitamin D levels in infertile women compared to fertile controls and to evaluate the association between vitamin D status, infertility and selected metabolic parameters. Methods: A hospital-based case–control study was conducted in Baghdad from March 2024 to February 2026, including 200 women (100 infertile cases and 100 fertile controls). Serum 25-hydroxyvitamin D levels were measured and classified as deficient (<20 ng/mL), insufficient (20–29 ng/mL), or sufficient (≥30 ng/mL). Fasting glucose, insulin, and insulin resistance (HOMA-IR) were also assessed. Statistical analysis included group comparisons, correlation analysis, and multivariate logistic regression. Results: Infertile women had significantly lower mean vitamin D levels compared to controls (17.2 ± 6.8 vs 26.5 ± 7.4 ng/mL, p < 0.001). Vitamin D deficiency was more prevalent in cases (68%) than controls (32%) (p < 0.001). Vitamin D deficiency was strongly associated with infertility (OR = 4.25, 95% CI: 2.20–8.21, p < 0.001) and remained an independent predictor after adjustment (adjusted OR = 3.87, 95% CI: 1.95–7.68, p < 0.001). Infertile women also exhibited significantly higher fasting glucose, insulin levels, and HOMA-IR (p < 0.001). Serum vitamin D levels showed significant negative correlations with HOMA-IR (r = –0.41, p < 0.001), insulin (r = –0.36, p < 0.001), BMI (r = –0.28, p = 0.004), and fasting glucose (r = –0.25, p = 0.009). Conclusion: Vitamin D deficiency is significantly associated with infertility and may contribute to reproductive dysfunction through metabolic pathways, particularly insulin resistance. Screening and correction of vitamin D deficiency may represent a simple and cost-effective strategy in the management of infertility.