Maternal vitamin D status, fetal growth patterns, and adverse pregnancy outcomes in a multisite prospective pregnancy cohort.

BACKGROUND

Few studies have examined maternal vitamin D status and fetal growth patterns across gestation. Furthermore, time points in pregnancy at which maternal vitamin D status is most critical for optimal fetal growth and pregnancy outcomes are uncertain.

OBJECTIVES

Our objective was to examine whether first and second trimester maternal vitamin D status are associated with fetal growth patterns and pregnancy outcomes.

METHODS

We conducted a secondary analysis using data and samples from a multisite prospective cohort study of nulliparous pregnant females in the United States. We measured serum 25-hydroxyvitamin D (25(OH)D) for 351 participants at 6-13 and 16-21 weeks of gestation. Fetal growth was measured by ultrasound at 16-21 and 22-29 weeks of gestation, and neonatal anthropometric measures at birth. We constructed fetal growth curves using length, weight, and head circumference z-scores, and calculated risk of preterm birth (<37 wk) and small for gestational age (SGA). We examined outcomes across 25(OH)D concentrations assessed continuously, using Institute of Medicine (IOM) cutoffs (<50 compared with ≥50 nmol/L), and using exploratory cutoffs (<40, 40-59.9, 60-79.9, ≥80 nmol/L).

RESULTS

Vitamin D insufficiency (25(OH)D <50 nmol/L) was prevalent in 20% of participants in the first trimester. Each 10 nmol/L increase in first trimester 25(OH)D was associated with a 0.05 [95% confidence interval (CI): 0.01, 0.10] increase in length-for-age z-score but was not associated with weight or head circumference. There were no differences in risk of preterm birth or SGA using IOM cutoffs; participants with first trimester 25(OH)D <40 compared with ≥80 nmol/L had 4.35 (95% CI: 1.14, 16.55) times risk of preterm birth. Second trimester 25(OH)D was not associated with fetal growth patterns or with pregnancy outcomes.

CONCLUSIONS

First trimester 25(OH)D is positively associated with linear growth. Low first trimester 25(OH)D (<40 nmol/L) is associated with a higher risk of preterm birth. Second trimester 25(OH)D is not associated with fetal growth or pregnancy outcomes assessed.