Thompson WD, et al. - Via two-sample mendelian randomization (MR) on 190,406 women, the researchers combined genetic and randomized controlled trials (RCTs) data to determine causal effects of vitamin D (25[OH]D) and calcium supplementation, the two maternal attributes on offspring birth weight, since they are at an increased risk of neonatal mortality and morbidity, in order to reduce the number of babies born with lower or higher than optimal birth weight. A strong effect of maternal 25(OH)D on birth weight wasn't evident. Also, the inconsistency of the proof of the effects of maternal calcium on birth weight was also noticed. Hence, no influence on birth weight in otherwise healthy newborns of maternal circulating 25(OH)D was observed. However, further research through RCT and/or MR analyses with more valid instruments on the effect of maternal circulating calcium on birth weight is suggestive.