Trial of Vitamin D Supplements to Raise Calcidiol Levels of Pregnant Women in Mongolia

Vitamin D deficiency (serum 25(OH)D levels \<20 ng/ml or 50 nmol/l) is widespread among Mongolians and is nearly universal during the winter months. The investigators have conducted a series of studies which have shown nearly universal Vitamin D deficiency in the winter months. Observational studies in the U.S. have linked low 25(OH)D levels in women to a higher risk of preeclampsia. Mongolians also report levels of preeclampsia that are 3 times higher than those in the U.S (official reports cite 15%, though this may include gestational hypertension). Given the widespread Vitamin D deficiency in Mongolian women, the high prevalence of hypertensive pregnancy in Mongolia, and the observational studies linking low 25(OH)D levels with preeclampsia risk, the investigators are conducting a pilot Vitamin D dosing trial to: * Determine what level of Vitamin D supplementation is needed to secure 25(OH)D levels of at least 20 ng/ml in pregnant Mongolian women * Generate preliminary data in support of a funding application for a larger trial of Vitamin D supplements to prevent preeclampsia in Mongolia A double-blind randomized, placebo-controlled trial comparing the impact of 600 IU, 2000 IU, or 4000 IU of Vitamin D3 on third trimester 25(OH)D levels and change from baseline. The Vitamin D will be integrated in a standard prenatal vitamin, which will be taken from 12-16 weeks' gestation and continue throughout pregnancy. Umbilical cord 25(OH)D levels will also be determined. The investigators will generate preliminary data regarding Vitamin D intake and hypertensive disorders, blood pressure, and arterial function measured by tonography. The investigators will independently test blood pressure and proteinuria to identify preeclampsia cases. The investigators will enroll 360 women who plan to receive their prenatal care and deliver at Zuun Kharaa Hospital in the Selenge province in northern Mongolia. Enrollment will be stratified by season. 120 women will be randomized to each of the three doses of vitamin D (600, 2000, 4000 IU) included in a standard prenatal vitamin. Calcemia will be monitored two months after randomization and weeks 36-40 of pregnancy. Adverse events of pregnancy will be reported to the Data Safety and Monitoring Board (DSMB) and Institutional Review Board (IRB) per protocol. Data will be gathered by a Mongolian study coordinator and local clinicians. A DSMB comprised of a U.S. and Mongolian clinicians and statisticians will monitor study data for safety.