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.