New findings on vitamin D are important for every pregnant woman. A study found that complications of pregnancy, such as preterm labor, preterm birth, and infection were lowest in women with the highest vitamin D levels.
 
Blood levels of activated Vitamin D usually rise during very early pregnancy, and some of it crosses the placenta to bathe the fetus, especially the developing fetal brain, in activated vitamin D. But many -- in fact most -- pregnant women do not make as much vitamin D as they need.

4,000 IU of vitamin D per day during pregnancy was found to be safe (not a single adverse event).  However, this amount only resulted in a mean vitamin D blood level of 27 ng/ml in the newborn infants, indicating that even 4,000 IU per day during pregnancy is not enough.