Maternal 25(OH)D concentrations ≥40 ng/mL associated with 60% lower preterm birth risk among general obstetrical patients at an urban medical center.

Background: Given the high rate of preterm birth (PTB) nationwide and data from RCTs demonstrating risk reduction with vitamin D supplementation, the Medical University of South Carolina (MUSC) implemented a new standard of care for pregnant women to receive vitamin D testing and supplementation.
Objectives: To determine if the reported inverse relationship between maternal 25(OH)D and PTB risk could be replicated at MUSC, an urban medical center treating a large, diverse population.
Methods: Medical record data were obtained for pregnant patients aged 18-45 years between September 2015 and December 2016. During this time, a protocol that included 25(OH)D testing at first prenatal visit with recommended follow-up testing was initiated. Free vitamin D supplements were offered and the treatment goal was ≥40 ng/mL. PTB rates (<37 weeks) were calculated, and logistic regression and locally weighted regression (LOESS) were used to explore the association between 25(OH)D and PTB. Subgroup analyses were also conducted.
Results: Among women with a live, singleton birth and at least one 25(OH)D test during pregnancy (N = 1,064), the overall PTB rate was 13%. The LOESS curve showed gestational age rising with increasing 25(OH)D. Women with 25(OH)D ≥40 ng/mL had a 62% lower risk of PTB compared to those <20 ng/mL (p<0.0001). After adjusting for socioeconomic variables, this lower risk remained (OR = 0.41, p = 0.002). Similar decreases in PTB risk were observed for PTB subtypes (spontaneous: 58%, p = 0.02; indicated: 61%, p = 0.006), by race/ethnicity (white: 65%, p = 0.03; non-white: 68%, p = 0.008), and among women with a prior PTB (80%, p = 0.02). Among women with initial 25(OH)D <40 ng/mL, PTB rates were 60% lower for those with ≥40 vs. <40 ng/mL on a follow-up test (p = 0.006); 38% for whites (p = 0.33) and 78% for non-whites (p = 0.01).
Conclusions: Maternal 25(OH)D concentrations ≥40 ng/mL were associated with substantial reduction in PTB risk in a large, diverse population of women.