Clinical Review

Vitamin D and pregnancy: 9 things you need to know

OBG Manag. 2011 August;23(8):30-36

References

1. Dror DK, Allen LH. Vitamin D inadequacy in pregnancy: biology outcomes, and interventions. Nutr Rev. 2010;68(8):465-477.

2. Verstuyf A, Carmeliet G, Bouillon R, Mathieu C. Vitamin D: a pleiotropic hormone. Kidney Int. 2010;78(2):140-145.

3. Merewood A, Mehta SD, Chen TC, Bauchner H, Holick MF. Association between vitamin D deficiency and primary cesarean section. J Clin Endocrinol Metab. 2009;94(3):940-945.

4. Bodnar LM, Krohn MA, Simhan HN. Maternal vitamin D deficiency is associated with bacterial vaginosis in the first trimester of pregnancy. J Nutr. 2009;139(6):1157-1161.

5. Robinson CJ, Alanis MC, Wagner CL, Hollis BW, Johnson DD. Plasma 25-hydroxyvitamin D levels in early-onset severe preeclampsia. Am J Obstet Gynecol. 2010;203(4):366.e1-6.

6. Lau SL, Gunton JE, Athayde NP, Byth K, Cheung NW. Serum 25-hydroxyvitamin D and glycated haemoglobin levels in women with gestational diabetes mellitus. Med J Aust. 2011;194(7):334-337.

7. Mulligan ML, Felton SK, Riek AE, Bernal-Mizrachi C. Implications of vitamin D deficiency in pregnancy and lactation. Am J Obstet Gynecol. 2010;202(5):429.e1-9.

8. Hollis BW. Vitamin D requirement during pregnancy and lactation. J Bone Miner Res. 2007;22 (suppl 2):V39-44.

9. Johnson DD, Wagner CL, Hulsey TC, McNeil RB, Ebeling M, Hollis BW. Vitamin D deficiency and insufficiency is common during pregnancy. Am J Perinatol. 2010;28(1):7-12.

10. Bodnar LM, Simhan HN, Powers RW, Frank MP, Cooperstein E, Roberts JM. High prevalence of vitamin D insufficiency in black and white pregnant women residing in the northern United States and their neonates. J Nutr. 2007;137(2):447-452.

11. Ginde AA, Sullivan AF, Mansbach JM, Camargo CA, Jr. Vitamin D insufficiency in pregnant and nonpregnant women of childbearing age in the United States. Am J Obstet Gynecol. 2010;202(5):436.e1-8.

12. ACOG Committee Opinion No. 495: Vitamin D: Screening and supplementation during pregnancy. Obstet Gynecol. 2011;118(1):197-198.

13. Wagner CL, Greer FR. American Academy of Pediatrics Section on Breastfeeding; American Academy of Pediatrics Committee on Nutrition. Prevention of rickets and vitamin D deficiency in infants children, and adolescents. Pediatrics. 2008;122(5):1142-1152.

14. Yu CK, Sykes L, Sethi M, Teoh TG, Robinson S. Vitamin D deficiency and supplementation during pregnancy. Clin Endocrinol (Oxf). 2009;70(5):685-690.

15. Jones G. Pharmacokinetics of vitamin D toxicity. Am J Clin Nutr. 2008;88(2):582S-586S.

16. Hathcock JN, Shao A, Vieth R, Heaney R. Risk assessment for vitamin D. Am J Clin Nutr. 2007;85(1):6-18.

17. Hollis BW. Vitamin D requirement during pregnancy and lactation. J Bone Miner Res. 2007;22 (suppl 2):V39-44.

5. What level signals deficiency?

Experts disagree about the level of vitamin D that signals deficiency. Many labs report a reference range of 32 to 100 ng/mL as normal. However, in November 2010, the Institute of Medicine (IOM) weighed in on the matter. After examining the data, the IOM suggested that a vitamin D level of 20 ng/mL is sufficient to prevent bone loss and changes seen in rickets and osteoporosis.

This level is hotly contested by experts in other fields, who argue that, although 20 ng/mL may be considered the bare minimum level to prevent negative bone resorption changes, it can hardly be construed as a normal level.

Nor did the IOM recommendation take pregnancy into consideration. Therefore, the IOM made no comment as to whether a level of 20 ng/mL is sufficient for a pregnant woman, given that the fetus will be actively soliciting maternal vitamin D for its own development. Indeed, some researchers have indicated that the actual daily recommended intake for pregnancy and lactation may be as high as 6,000 IU/day.⁸

6. How many women are deficient?

The rate of deficiency varies, but studies have documented rates as high as 97% in some pregnant populations; the rates vary by race and latitude.^9-11

The high prevalence of deficiency in the population is due, in large part, to vitamin D’s mode of production and changes in human lifestyle and culture. Vitamin D is produced primarily through direct exposure of the skin to the sun. Over the past 50 years, as more and more people have come to spend their days in an office or factory instead of on a farm, the opportunity to produce vitamin D has greatly diminished.

Other entities or practices that reduce the production of vitamin D:

Sunscreen SPF 50 may prevent skin cancer, but it also blocks vitamin D production.
Fat cells Obese patients produce vitamin D less rapidly than patients of normal weight.
Melanin Darker-skinned people produce vitamin D at a slower rate than those who have fair skin.
Cultural practices Some religious and cultural practices mandate full skin coverage in public, particularly for women, leading to minimal sun exposure.
Age Older people also produce vitamin D more slowly. Among the population of reproductive age, however, the effect of age is minimal.
Latitude Northern latitudes, with their longer winters and shorter summers, provide less opportunity for sun exposure.

Because vitamin D is, in essence, a “seasonal” vitamin, it makes evolutionary sense that the human body has developed a wide normal range to “store up” vitamin D when sunshine is plentiful and then use its stores during times of scarcity, such as winter. This seasonal variability is another reason why the rate of deficiency can vary, depending on the time and location of study.

Because vitamin D deficiency is clinically silent until severe events such as rickets occur, the best way to check for it is to measure total levels of the two forms of vitamin D found in the body—D₂ and D₃. The recommended test is total 25-hydroxy vitamin D (25-OHD). Measurement of the activated form of vitamin D—1,25-OHD—will not tell you whether a person’s overall stores are lacking, because the body maintains a normal 1,25-OHD level over a wide range until severe deficiency occurs.

7. Should you test all pregnant patients for deficiency?

ACOG does not recommend that vitamin D be measured routinely in pregnant women.¹² In a Committee Opinion published in July 2011, ACOG determined that “there is insufficient evidence to support a recommendation for screening all pregnant women for vitamin D deficiency.”¹²

Many experts disagree, however, citing the increased rate of rickets being found in the United States.^6,8 Pediatricians in the United States have found such a high rate of deficiency in the neonatal population that the American Academy of Pediatricians now recommends that all exclusively breastfed babies be given a supplement of 400 IU of vitamin D daily, beginning in the first few days of life.¹³

ACOG acknowledged that, for pregnant patients “thought to be at increased risk, measurement of total levels can be considered with “high-risk groups” that have many of the risk factors cited earlier.¹²

If you want to test your patients, no single plan is recommended. A sample algorithm includes the following steps:

Measure total 25-OHD at the time of prenatal registration labs
Select a level of supplementation, based on the findings (see TABLE 2)
Recheck the 25-OHD level after 3 months. For most patients, this would be around the time of a standard glucose screening test
Adjust the supplementation level, as needed
Measure 25-OHD at admission to labor and delivery.