High prevalence of severe vitamin D deficiency during the first trimester in pregnant women in Switzerland and its potential contributions to adverse outcomes in the pregnancy
Authors: Christoph Patricia , Challande Pauline , Raio Luigi , Surbek Daniel 
Published: June 2020

PURPOSE
Vitamin D is primarily known for its role in bone health. However, it has a much more diverse role in the human metabolism. Specifically, deficiency of vitamin D has recently been studied for its possible role in adverse pregnancy outcomes such as preeclampsia, gestational diabetes and preterm birth. Vitamin D levels largely depend on exposure to the sun and are influenced by nutritional habits at only a minimal level. In Switzerland, it is estimated that 40–50% of the population is vitamin D deficient. No specific data on pregnant women is available. The recommendations of the Swiss Federal Commission for Nutrition include a supplement of 600 IU of vitamin D to all pregnant women, despite the lack of data for this population in Switzerland. The primary aim of this study was to determine the prevalence of vitamin D deficiency among the population of pregnant women receiving prenatal care and giving birth at our clinic. We assumed that the prevalence of vitamin D deficiency in pregnant women in Switzerland is significantly higher than what has been estimated. Therefore, the current recommendations for vitamin D supplementation in pregnant women may be insufficient to achieve appropriate vitamin D levels. Furthermore, we aimed to address the issue of the potential influence of vitamin D deficiency on adverse pregnancy outcomes.

METHODS
We performed a retrospective, observational cross-sectional study of 1382 pregnant women attending prenatal care at our department between 2012 and 2015. Serum 25-dihydroxycholecalciferol (25(OH)D) levels were determined in the first trimester, and the patient’s characteristics, the course of the pregnancy, any complications, the delivery and the neonatal outcome were analysed. The risk factors for vitamin D deficiency and its correlation with adverse pregnancy outcomes were assessed using a multivariate analysis.

RESULTS
The clear majority (73.23%) of the population studied were found to be vitamin D deficient, with serum levels of 25(OH)D <50 nmol/l. More importantly, severe vitamin D deficiency (25(OH)D levels below 25 nmol/l) was present in one third (34.2%) of all pregnant women. The mean 25(OH)D level was 36.72 ± 19.63 nmol/l. In the multivariate analysis, those with a high BMI and who belonged to ethnicities comprising people who are generally dark-skinned were found to be associated with lower 25(OH)D serum levels (p <0.0001). We detected a seasonal influence: the mean 25(OH)D level was significantly higher during the summer season (April–September) compared to the winter season (October–March) (p <0.0001). We found an association between low 25(OH)D serum level and gestational diabetes (p = 0.0116). Surprisingly, a low 25(OH)D level was also associated with decreased incidence of postpartum hemorrhage and placental retention (p = 0.02). We found no association between the 25(OH)D serum level and preeclampsia, preterm birth, postdate pregnancy, miscarriage, intrauterine growth restriction, bacterial vaginosis, mode of delivery, or neonatal birth weight and length.

CONCLUSION
We performed a retrospective analysis of serum 25(OH)D concentrations in pregnant Swiss women and found a mean serum 25(OH)D level of about 37 nmol/l and that one third of the overall study population had a serum 25(OH)D level below 25 nmol/l, and were thus seriously vitamin D deficient. Furthermore, the data demonstrate that vitamin D deficiency is associated with gestational diabetes. The current recommendations of vitamin D supplementation of 600 IU in pregnant women are therefore insufficient, and novel strategies, such as general screening for vitamin D deficiency, pre-conceptional timing of the supplementation and individually tailored dosing of vitamin D supplementation seem mandatory, potentially leading to improved maternal health and benefits to children’s long-term health in Switzerland and worldwide. (trial registration ClinicalTrial.gov. Identifier: NCT02904720)