Timing over Dose: Maternal Vitamin D, Periconceptional Window, and Early-Life Respiratory Programming

Background: Vitamin D deficiency affects an estimated 40–60% of pregnant women worldwide and is associated with adverse obstetric and neonatal outcomes. Childhood asthma, the most prevalent chronic paediatric disease, has emerged as a plausible programming target, since vitamin D regulates foetal lung branching morphogenesis, calibrates the developing immune system, and modulates decidual and placental function in early gestation. Two landmark randomised trials, VDAART (intervention from weeks 10–18) and COPSAC2010 (from week 24), each reported a 20–25% reduction in offspring asthma or recurrent wheeze at age 3, yet neither reached significance in primary analysis, and the protective signal attenuated by school age. Post hoc stratification by baseline maternal 25-hydroxyvitamin D [25(OH)D] and 17q21 genotype recovered significant effects, raising the possibility that population-average nulls conceal a real but modifier-conditional benefit. Aim: This narrative review re-examines the evidence through a developmental-timing lens, arguing that the periconceptional and first-trimester window, rather than mid-gestation, is the biologically relevant interval for any protective effect. Methods: The review utilises a narrative synthesis of randomised trials, birth-cohort studies, mechanistic investigations, and recent meta-analyses (PubMed, Embase, Cochrane Library to April 2026) relevant to maternal vitamin D, placental biology, and offspring asthma. Findings: The periconceptional weeks coincide with implantation, decidualisation, the embryonic and pseudoglandular phases of airway morphogenesis, and the onset of epigenetic programming, while decidual CYP27B1 expression is prominent in the first trimester. Both trials initiated supplementation after branching morphogenesis was largely complete. Effect modifiers, including baseline 25(OH)D, vitamin D-binding protein, and maternal 17q21 genotype, indicate substantial inter-individual heterogeneity masked in unselected populations. Conclusions: Repositioning preventive supplementation toward the preconceptional and first-trimester window, stratified by baseline status, offers a biologically coherent strategy that existing mid-pregnancy trials have not tested. Adequately powered preconceptional trials with serial biomarker measurement and objective respiratory phenotyping are the priority.