OBJECTIVE: To assess the effect of 25-hydroxyvitamin D (25-OHD) levels on 
pregnancy outcomes and birth variables.
DESIGN: Systematic review and meta-analysis.
DATA SOURCES: Medline (1966 to August 2012), PubMed (2008 to August 2012), 
Embase (1980 to August 2012), CINAHL (1981 to August 2012), the Cochrane 
database of systematic reviews, and the Cochrane database of registered clinical 
trials.
STUDY SELECTION: Studies reporting on the association between serum 25-OHD 
levels during pregnancy and the outcomes of interest (pre-eclampsia, gestational 
diabetes, bacterial vaginosis, caesarean section, small for gestational age 
infants, birth weight, birth length, and head circumference).
DATA EXTRACTION: Two authors independently extracted data from original research 
articles, including key indicators of study quality. We pooled the most adjusted 
odds ratios and weighted mean differences. Associations were tested in subgroups 
representing different patient characteristics and study quality.
RESULTS: 3357 studies were identified and reviewed for eligibility. 31 eligible 
studies were included in the final analysis. Insufficient serum levels of 25-OHD 
were associated with gestational diabetes (pooled odds ratio 1.49, 95% 
confidence interval 1.18 to 1.89), pre-eclampsia (1.79, 1.25 to 2.58), and small 
for gestational age infants (1.85, 1.52 to 2.26). Pregnant women with low serum 
25-OHD levels had an increased risk of bacterial vaginosis and low birthweight 
infants but not delivery by caesarean section.
CONCLUSION: Vitamin D insufficiency is associated with an increased risk of 
gestational diabetes, pre-eclampsia, and small for gestational age infants. 
Pregnant women with low 25-OHD levels had an increased risk of bacterial 
vaginosis and lower birth weight infants, but not delivery by caesarean section.