Maternal vitamin D status

Vitamin D status was determined based on serum 25(OH)D analysis. A venous blood sample (2ml) was collected from the respondents. The serum samples were sent to the laboratory and analysed using the Siemens ADVIA Centaur Vitamin D Total assay (Siemens, Tarrytown, NY, USA). This assay has been standardized to the University of Ghent ID-LC/MS/MS reference measurement procedure [22] and has achieved the Centers for Disease Control Vitamin D Standardization Certification [23]. Serum 25(OH)D level was classified into vitamin D deficiency (< 30 nmol/L), vitamin D insufficiency (30–50 nmol/L) and vitamin D sufficient (≥ 50 nmol/L) [24].