From the Vitamin D Council:
December 13, 2012 — John Cannell, MD
Professors Carole Wagner and Bruce Hollis and ten of their colleagues at the Medical University of South Carolina recently conducted the largest randomized controlled trial to date using meaningful daily doses (2,000 vs. 4,000 IU) of vitamin D during pregnancy.
Wagner CL, McNeil R, Hamilton SA, Winkler J, Cook CR, Warner G, Bivens B, Davis DJ, Smith PG, Murphy M, Shary J, Hollis BW. A Randomized Trial of Vitamin D Supplementation in Two Community Health Center Networks in South Carolina. Am J Obstet Gynecol. 2012 Nov 3.
The ethics committee would not let them use a control group of 400 IU/day, as the committee felt this would endanger the women and their newborns. Sadly, most women in this country only take the 400 IU/day in their prenatal vitamin.
The researchers randomized 256 pregnant women, 160 of whom completed the study. They were separated into two groups, 2,000 or 4,000 IU per day, beginning at 3-4 months of pregnancy. They followed the 160 women through delivery and found the following:

1. At the beginning of the study, pregnant African American women had a level of 18.5 ng/ml, while Whites had notably higher mean values of 29.5 ng/ml.
2. Neither group had any side effects; in fact the blood calcium levels of the 4,000 IU group actually went down.
3. At delivery, the average cord blood vitamin D level was 22 ng/ml in the 2,000 IU/day group and 27 ng/ml in the 4,000 IU/day group, still slightly less than cord levels of hunter gatherers.
4. Overall, only 37% of the 2,000 IU group and 46% of the 4,000 IU group achieved vitamin D levels of 40 ng/ml by the end of their pregnancies. Furthermore, 40 ng/ml was the threshold level at which conversion of 25(OH)D to activated vitamin D finally flattened out during pregnancy. In other words, the more 25(OH)D the pregnant woman had, the higher the activated vitamin D level, until a 25(OH)D level of 40 ng/ml was reached, and activated vitamin D stopped increasing any more at higher levels.
5. The 4,000 IU group participants had 2.40 times higher odds of having an infant in the 50th percentile of birth weight, compared to the 2,000 IU group. That is, the 4,000 IU group was more likely to have normal weight babies.
6. Lower pre‐delivery 25(OH)D was significantly predictive of preterm delivery (p=0.001)
7. Lower pre‐delivery 25(OH)D was associated with more infection (p=0.026).
8. Overall, complications of pregnancy were less with the 4,000 IU/day group than with the 2,000 IU/day group, though not statistically significant.

In my opinion, this is once again great research that shows that pregnant women should have levels over 40 ng/ml, and I think it’s preferable to target levels between 50-60 ng/ml.
This is why the Council recommends 6,000 IU/day, to ensure these kinds of levels. We also recommend checking 25(OH)D levels periodically throughout pregnancy to make sure your 25(OH)D levels remain in the natural range, about 50-60 ng/ml. Some women may need more than 6,000 IU/day.