Researchers at the USDA Western Human Nutrition Research Center and clinicians at UC Davis Medical Center are currently conducting a study on the effects of vitamin D supplementation during pregnancy on maternal health and immune function. The investigators believe that vitamin D supplementation at levels greater than the current recommendation is beneficial for maternal health during pregnancy. In particular, there is evidence that vitamin D deficiency during pregnancy puts women at greater risk for diseases like pre-eclampsia, which may be due to an imbalance in immune function at the placenta. This randomized, double-blind study is designed to test this hypothesis. Pregnant women who participate in the study receive either 400 IU or 2,000 IU of vitamin D per day for the duration of their pregnancy. The investigators will collect blood at three time points during pregnancy (16-20 weeks, 26-28 weeks, 36 weeks gestation) for analysis of immune function and vitamin D status. The investigators are also collecting data on clinical parameters, such as blood pressure and glucose tolerance.

Inclusion Criteria:

• Pregnant women
• Less than 20 weeks gestation
• Over 18 years of age
• Able to swallow nutritional supplements

Exclusion Criteria:

• Use of medications known to affect vitamin D metabolism
• Diagnosis of type 1 diabetes
• History of thyroid, renal, or liver disease
• Problems with digestion or absorption