AbstractSTUDY QUESTIONIs preconception vitamin D level associated with the risk of miscarriage?SUMMARY ANSWERPreconception vitamin D levels are not associated with the risk of miscarriage in a population of women conceiving naturally.WHAT IS KNOWN ALREADYIn humans, low vitamin D has been associated with prolonged menstrual cycles, delayed ovulation and a lower probability of conception. Animal and in vitro data indicate that vitamin D may affect implantation.STUDY DESIGN, SIZE, DURATIONThis prospective time-to-pregnancy study included 362 women who were trying to conceive naturally between 2008 and 2015.PARTICIPANTS/MATERIALS, SETTING, METHODSThis study included participants who had been trying to conceive naturally for 3 months or less at enrollment and aged 30–44 years. A preconception blood sample was collected and 25-hydroxyvitamin D [25(OH)D] was measured. Women who conceived (N = 362) were at risk of a miscarriage from the day of a reported positive pregnancy test until either a participant-reported pregnancy loss or 20 weeks post day of last menstrual period, whichever came first. Gestational age was defined by ovulation. Time to miscarriage (days) or censoring was modeled using a multivariate Cox proportional hazards model. Multiple imputation was performed for missing covariates and missing day of ovulation.MAIN RESULTS AND THE ROLE OF CHANCEThe mean age was 33 years (SD: 3.0 years). Mean 25(OH)D was lower among those who reported their race as African-American and those with a higher BMI. After adjustment for age, race, BMI, education, exercise, alcohol and caffeine intake, compared to the referent group (30–<40 ng/ml), the hazard ratio (HR) and 95% CI for those with a low 25(OH)D level (<30 ng/ml) was 1.10 (CI: 0.62, 1.91). Among participants with a higher 25(OH)D level (≥40 ng/ml), the HR was 1.07 (CI: 0.62, 1.84).LIMITATIONS, REASONS FOR CAUTIONThis study was limited by a 25(OH)D measurement at only a single time point. A large percentage of women in this study had sufficient vitamin D levels, which may have limited our power to detect an effect of deficiency. Women in this study were older (30–44 years), and predominantly reported their race as White which may limit generalizability.WIDER IMPLICATIONS OF THE FINDINGSThe findings of this study do not suggest an association between preconception vitamin D and miscarriage. Future research should focus on women at greater risk for miscarriage or in populations at risk for vitamin D deficiency or on supplementation.STUDY FUNDING/COMPETING INTEREST(S)This research was supported in part by the Intramural Research Program of the NIH, National Institute of Environmental Health Sciences (Z01ES103333). This research was also supported by the Eunice Kennedy Shriver National Institute of Child Health and Human Development of the National Institutes of Health (NIH) under award numbers R00HD079659 and R01HD067683. The authors have no conflicts of interest.TRIAL REGISTRATION NUMBERN/A.