Background: Infertility and fecundability problems have been linked with lower 25-hydroxyvitamin D (25(OH)D) concentrations, but studies conducted with small, heterogenous or selected populations have shown inconsistent results. Methods: This study included women at age 31 from prospective population-based Northern Finland Birth Cohort 1966. Serum 25(OH)D concentrations were evaluated between women with or without previous infertility examinations or treatments (infertility group,n= 375, reference group,n= 2051) and time to pregnancy (TTP) of over 12 months (decreased fecundability group,n= 338) with a wide range of confounders. Furthermore, 25(OH)D concentrations were also compared among reproductive outcomes. Results: The mean 25(OH)D concentration was lower and 25(OH)D < 30 nmol/L was more frequent in women with a history of infertility compared to reference group. Moreover, 25(OH)D > 75 nmol/L was more frequent in the reference group. The mean 25(OH)D concentration was lower in women who had had multiple miscarriages. Both history of infertility (β = −2.7, 95% confidence interval (CI) −4.6, −0.7) and decreased fecundability associated with lower 25(OH)D concentration (β = −4.1, 95% CI −7.4, −0.8) after adjustments. In conclusion, this population-based study demonstrated that previous infertility and decreased fecundability were associated with lower 25(OH)D.