Pregnant Women and Vitamin D: Researchers say taking vitamin D supplements during pregnancy can prevent preterm labor/births and infections. Vitamin D is good for the immune system and bone health. The greatest effects were seen among women taking 4,000 IU of vitamin D per day. Why It Matters: Vitamin D is important for maternal and infant health. Vitamin D deficiency during pregnancy is, however, a serious public health problem. Researchers say diet and sun do not provide the amount of vitamin D needed.