I wanted to pull this section out of
(Today's Dietitian) just in case you missed the whole article in the research section.
Pregnancy Along with the general population, pregnant women have been found to have a high incidence of vitamin D deficiency.7 Vitamin D status of the infant at birth is related to the vitamin D status of the mother, as the cord blood will contain 50% to 60% of the maternal circulating concentrations of vitamin D.20
A vitamin D deficiency in pregnant women can cause problems for the mother and the fetus. Maternal effects of vitamin D deficiency include decreased serum calcium concentrations and possible decreased weight gain in the third trimester.21 Fetal vitamin D deficiency can delay growth and bone ossification and cause enamel hypoplasia and problems with calcium regulation (ie, hypocalcemia and tetany), decrease bone mineral content and skeletal mineralization, and cause congenital rickets and craniotabes (softening of the skull).21,22
At issue is whether a vitamin D deficiency decreases maternal weight gain, fetal growth, and birth weight and whether the serum vitamin D level for pregnant women should be higher.
In a study of pregnant women in the Netherlands, 8% of light-skinned women and more than 50% of darker pigmented women were vitamin D deficient, with levels below 25 nmol/L (a level that causes osteomalacia in adults).22 Had the criteria for vitamin D deficiency been set higher, an even greater number of pregnant women would have been identified as vitamin D deficient—up to 100% of the women in the study.
A 2006 study found a correlation of maternal milk intake to infant birth weight in pregnant women living in Calgary, Canada (51º North latitude), independent of other risk factors. As milk intake increased, so did birth weight.23 For each microgram (40 IU) increase in vitamin D intake, birth weight increased by 11 grams. No differences in infant head circumference and length were found between women with higher and lower milk consumption. No serum vitamin D levels were taken.
A study of lactating women given either 2,000 or 4,000 IU per day for three months increased serum vitamin D from 69 to 90.25 nmol/L and 81.5 to 97.9 nmol/L, respectively.20 All these values were within the normal reference range. In addition, the infants of the lactating women had significantly better vitamin D status at the end of the study, since their breast milk was richer in vitamin D. It appears supplementation does improve vitamin D status in pregnant women and their infants, and a vitamin D supplement up to 4,000 IU per day is not harmful.20