Background Each year, an estimated 15 million babies are born preterm. Micronutrient deficiencies, including vitamin D deficiency (VDD), are common in many low- and middle-income countries (LMICs), and these conditions are often associated with adverse pregnancy outcomes. Bangladesh experiences a high prevalence of VDD. The country also has a high preterm birth (PTB) rate. Using data from a population-based pregnancy cohort, we estimated the burden of VDD during pregnancy and its association with PTB. Methods Pregnant women (N = 3,000) were enrolled after ultrasound confirmation of gestational age at 8–19 weeks of gestation. Trained health workers prospectively collected phenotypic and epidemiological data at scheduled home visits. Trained phlebotomists collected maternal blood samples at enrollment and 24 -28 weeks of gestation. Aliquots of serum were stored at -800 C. We conducted a nested case–control study with all PTB (n = 262) and a random sample of term births (n = 668). The outcome, PTB, was defined as live births < 37 weeks of gestation, based on ultrasound. The main exposure was vitamin D concentrations of 24–28 weeks maternal blood samples. The analysis was adjusted for other PTB risk factors. Women were categorized as VDD (lowest quartile of 25(OH)D; <  = 30.25 nmol/L) or not deficient (upper-three quartiles of 25(OH)D; > 30.25 nmol/L). We used logistic regression to determine the association of VDD with PTB, adjusting for potential confounders. Results The median and interquartile range of serum 25(OH)D was 38.0 nmol/L; 30.18 to 48.52 (nmol/L). After adjusting for co-variates, VDD was significantly associated with PTB [adjusted odds ratio (aOR) = 1.53, 95% confidence interval (CI) = 1.10 – 2.12]. The risk of PTB was also higher among women who were shorter (aOR = 1.81, 95% CI: 1.27–2.57), primiparous (aOR = 1.55, 95% CI = 1.12 – 2.12), passive smokers (aOR = 1.60, 95% CI = 1.09 – 2.34), and those who received iron supplementation during pregnancy (aOR = 1.66, 95% CI: 1.17, 2.37). Conclusion VDD is common in Bangladeshi pregnant women and is associated with an increased risk of PTB.

Background: Each year, an estimated 15 million babies are born preterm. Micronutrient deficiencies, including vitamin D deficiency, are common in many low- and middle-income countries (LMICs), and these conditions are often associated with adverse pregnancy outcomes. Bangladesh experiences a high prevalence of vitamin D deficiency. The country also has a high preterm birth (PTB) rate. Using data from a population-based pregnancy cohort, we estimated the burden of vitamin D deficiency during pregnancy and its effect on PTB. Methods: Pregnant women (N=3,000) were enrolled after ultrasound confirmation of gestational age at 8-19 weeks of gestation. Trained health workers prospectively collected detailed phenotypic and epidemiological data at scheduled home visits. Trained phlebotomists collected maternal blood samples at enrollment and at 24 -28 weeks of gestation. Aliquots of serum were stored at -800 C. We conducted a nested case-control study, with all PTB (n=262) and a random sample of term births (n=668). We used the following data: vitamin D concentrations of 24-28 weeks maternal blood samples, ultrasound-determined gestational age, data on pregnancy outcomes, and other PTB risk factors data. PTB was defined as the birth of a baby <37 weeks of gestation. Women were categorized as vitamin D deficient (lowest quartile of 25(OH)D; <=30.18 nmol/L) or not deficient (upper-three quartiles of 25(OH)D; >30.18 nmol/L). We used logistic regression to determine the effect of vitamin D deficiency on PTB, adjusting for potential confounders. Results: The median and interquartile range of serum 25(OH)D was 38.0 nmol/L; 30.18 to 48.52 (nmol/L). After adjusting for co-variates, serum 25(OH)D deficiency was significantly associated with PTB [adjusted odds ratio (aOR) = 1.47, 95% confidence interval (CI) = 1.06 – 2.04]. The risk of PTB was also higher in primiparous women (aOR = 1.54, 95% CI = 1.12 – 2.12), passive smokers (aOR = 1.55, 95% CI = 1.06 – 2.26), and those who didn’t receive iron supplementation during pregnancy (aOR, 95% CI: 0.59, 0.41- 0.83). Conclusion: Vitamin D deficiency is common in Bangladeshi pregnant women, and these women experience an increased risk of PTB. Improving maternal vitamin D status during pregnancy may significantly reduce the risk of PTB.