Does vitamin D supplementation in pregnancy decrease the risk of having a low-birth-weight infant?
In several small, low-quality trials, vitamin D supplementation in pregnancy was associated with a statistically nonsignificant trend toward a decreased risk of low birth weight. There is insufficient evidence to recommend routine vitamin D supplementation in pregnancy. (Strength of Recommendation: C, based on consensus, disease-oriented evidence, usual practice, expert opinion, or case series.)
Recent evidence supports a role for vitamin D supplementation in adults, particularly older adults,1 to reduce the incidence of osteoporotic fractures. Research also has found an association between low vitamin D levels in pregnant women and poor pregnancy outcomes,2,3 specifically low birth weight (i.e., less than 2,500 g [5 lb, 9 oz]). Although the reason for this association is unclear, it may be that women with normal levels of vitamin D also have better general nutrition, which contributes to normal fetal growth.
The authors of this Cochrane review analyzed the results of several small randomized trials of vitamin D supplementation in pregnancy. Three trials involving 463 women examined low birth weight as an outcome, and showed a statistically nonsignificant trend toward a reduced risk of having a low-birth-weight infant (risk ratio = 0.48; 95% confidence interval, 0.23 to 1.01). It does not appear that these results were adjusted for gestational age, raising the question of whether the observed effect may have represented an actual reduction in preterm births. Also, the included trials had several quality issues: a lack of information about the randomization process; missing data in some studies and failure to perform intention-to-treat analyses; and a lack of standardized vitamin D dosing. Many of the participants were of Asian and African descent, which may limit generalizability to other populations.
Therefore, although vitamin D supplementation in pregnancy looks encouraging as a means of preventing low birth weight, the evidence does not yet support routine supplementation. The American College of Obstetricians and Gynecologists (ACOG) recommends testing only pregnant women who are at increased risk of vitamin D deficiency (e.g., women with limited sun exposure, women with darker skin that limits absorption of vitamin D).4 If a woman's vitamin D levels are 20 ng per mL (50 nmol per L) or less, ACOG recommends vitamin D supplementation in a dosage of 1,000 to 2,000 IU daily.