Assessment of 25(OH)D
Non-fasted venous blood
Non-fasted venous blood samples were obtained on the day that the study medication was dispensed and at 34 weeks’ gestation. Serum was stored at −80°C. 25(OH)D concentration was assessed by chemiluminescence immunoassay (Liaison automated platform, Diasorin, Minnesota, USA). All samples were analysed in a single batch at the end of the study at Medical Research Council (MRC) Human Nutrition Research, Cambridge, UK. Within- and between-assay coefficients of variation were 4.1 and 6.1%. Details of assay performance and quality control through participation in DEQAS and calibration against the National Institute of Standards and Technology (NIST) standards are given elsewhere.