A study in the journal Lancet showed that 49% of pregnant women are deficient in Vitamin D. Lowered Vitamin D status in pregnancy has been linked with children having lower bone densities at age 9.
Another study looked at 741 pregnant women and found 70.6% were deficient in vitamin D (showing lab values less than 25nmol/L). Those women with the most severe deficiencies (less than 12.5nmol/L) were found to have a link with gestational diabetes.
Safe amounts of Vitamin D to supplement in pregnancy is 1000-2000IU/ day, however, it is best to ask your physician about getting your vitamin D levels checked.
Reference: 1. Lancet 2006;367:36-43 2. Diabetes Metab Res Rev 2007