Vitamin D Deficiency in Pregnancy Linked to Low Birth Weight

Troy Brown

December 13, 2012

Vitamin D levels in early pregnancy are closely related to birth weight, head circumference and ponderal index, but not placental growth, in term infants, according to a study published online November 16 and in the January 2013 issue of the Journal of Clinical Endocrinology & Metabolism.

Alison D. Gernand, PhD, MPH, a postdoctoral associate in the Department of Epidemiology at the University of Pittsburgh Graduate School of Public Health and School of Medicine in Pennsylvania, and colleagues measured maternal 25-hydroxyvitamin D [25(OH)D] levels to study the relationship between maternal 25(OH)D levels and newborn and placental weight.

Participants were mothers who took part in the Collaborative Perinatal Project, an observational cohort that was conducted in 12 medical centers in the United States from 1959 to 1965.

"Our study is an important contribution to the epidemiology evidence that maternal vitamin D status, especially in early pregnancy, may contribute to both pathological and physiological fetal growth, but not placental growth, in term infants," the authors write.

The mean maternal 25(OH)D was 51.3 ± 28.0 nmol/L. In 34.8% of women, it was less than 37.5 nmol/L, and in 55.9% of women it was less than 50 nmol/L. The incidence of babies who were small for gestational age (SGA) was high.

In a bivariate analysis, women who had 25(OH)D levels of 37.5 nmol/L or more had babies with higher birth weight, head circumference, and ponderal index when compared with those whose 25(OH) levels were less than 37.5 nmol/L, but there were no differences in placental weight and placental-to-fetal weight ratio.

The differences in birth weight and head circumference by vitamin D deficiency remained after adjustment for trimester at maternal blood draw, maternal race/ethnicity, prepregnancy body mass index, height, smoking, season, and study site.

Investigators observed a nonlinear relationship between 25(OH)D levels and birth weight and head circumference (P < .01).

Birth weight increased by 3.6 g (95% confidence interval [CI], 1.1 - 6.1 g), and head circumference increased by 0.010 cm (95% CI, 0.002 - 0.018 cm) per 1 nmol/L increase in maternal 25(OH)D level up to 37.5 nmol/L, leveling off after that.

The researchers found no relationship between 25(OH)D level and ponderal index, placental weight, or placental-to-fetal weight ratio in adjusted analyses, regardless of the way vitamin D was specified. These associations were unaffected by trimester of vitamin D assessment, maternal race/ethnicity, or infant sex.

The trimester of vitamin D assessment did affect the relationship between maternal 25(OH)D and the risk for SGA (P < .05).

After adjustment for maternal race/ethnicity, prepregnancy body mass index, height, smoking, season, and study site, maternal 25(OH)D levels of 37.5 nmol/L or greater in the first trimester were associated with almost half the risk for SGA compared with levels less than 37.5 nmol/L.

Each 1 SD increase in 25(OH)D lowered the risk for SGA by 34%. The researchers found no evidence of a nonlinear association, and no relationship existed between second trimester 25(OH)D level and SGA, regardless of the way vitamin D was specified.

"Randomized controlled trials that begin early in pregnancy are needed to provide causal evidence for clinical recommendations regarding vitamin D intake and potential screening in the care of pregnant women," the authors write.

This study was supported by the National Institutes of Health. The authors have disclosed no relevant financial relationships.

J Clin Endocrinol Metabol. Published online November 16, 2012. Abstract

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