Intervention
Participants were randomized to either cholecalciferol 1000 IU/day or matched placebo [Merck KGaA, (Darmstadt, Germany)/Sharp Clinical Services (Crickhowell, UK; previously DHP-Bilcare)], commenced before 17 weeks’ gestation. Packs of medication were randomly assigned in a 1:1 ratio by a computer-generated sequence in randomly permuted blocks of 10, starting randomly midway through the block, and sequentially numbered, before delivery to the study sites, and then dispensed in order by each study pharmacist. The study medication was provided in a single box containing all medication for the whole pregnancy. The participants, individuals providing antenatal and intrapartum care, and all field researchers involved in data collection and sample analysis were blinded to the assignment of the intervention. All participants received standard antenatal care, and could continue self-administration of dietary supplements containing up to 400 IU/day vitamin D. Women wishing to take dietary supplements containing >400 IU/day vitamin D were excluded from participation in the study, and those who increased their personal supplementation use above this threshold during the study were excluded from the analysis.