Thompson WD, et al. - Since systematic reviews of randomised controlled trials (RCTs) have recommended that maternal vitamin D (25[OH]D) and calcium supplementation increase birth weight, researchers sought to combine genetic and RCT data to estimate causal impacts of these two maternal traits on offspring birth weight. For this investigation, they analyzed genetic data on 190,406 women from the UK Biobank who reported the birth weight of their first child, along with the results from published studies of genetic associations with 25(OH)D and calcium levels in 122,123 and 61,275 people, respectively. No proof of a powerful maternal 25(OH)D impact on birth weight was discovered. In addition, the inconsistency of the proof of the effects of maternal calcium on birth weight was also noticed. Therefore, in otherwise healthy newborns of maternal circulating 25(OH)D, no influence on birth weight was observed. However, the impact of maternal circulating calcium on birth weight is uncertain and needs further studies with more valid tools including RCT and/or MR analyzes.