Insights from outside BJOG

Vitamin D supplementation in pregnancy Very few supplements in pregnancy, other than folic acid, have convincing evidence that they improve maternal or fetal outcomes in well-nourished women. Adding supplements and measuring their effects downstream is laborious and expensive in terms of time and resources. Research in this field also requires measurable endpoints that can be attributable to the intervention shortly after birth, then through infancy and childhood before environmental effects obfuscate results. Against this background, some recently published vitamin D antenatal studies are remarkable. They were randomised controlled trials of high-dose or regular-dose vitamin D starting in early pregnancy to investigate whether high-dose supplementation prevents asthma and promotes skeletal development in children. In the asthma trial, the initial set of results when the children were 3 years old had shown that there was some protection against respiratory malfunction but this was starting to wane by the third year. Now the investigators present their follow-up data of the entire cohort at 6 years of age (Litonjua et al. N Engl J Med 2020;382:525–33). They found that any early effect was not sustained, irrespective of maternal levels of vitamin D at study entry. The study had an elegant design and diligent follow-up, and epitomises the essential nature of final conclusions being reached, which were that high-dose vitamin D is of doubtful value beyond 3 years in preventing asthma or wheeze in children whose parents were susceptible to respiratory conditions (von Mutius and Martinez N Engl J Med 2020;382:574–75). There have been two further studies from Scandinavia assessing the effects of high-dose vitamin D supplementation in the second half of pregnancy and its outcomes in terms of the child’s skeletal health at 6 years of age. The first showed overall higher bone mineralisation status as measured by dual-energy X-ray absorptiometry scans, and this effect was most pronounced in offspring of vitamin D-deficient mothers and children born during the winter months (Brustad et al. JAMA Pediatr 2020. https://doi.org/10.1001/jamaped iatrics.2019.6083). The second looked at dental enamel defects in children in a similar age group and found that, if mothers received high-dose supplements, the children had fewer enamel defects, which they suggest will have benefit for future dental health (Nørrisgaard et al. JAMA Pediatr 2019;173:924–30). These outcomes are encouraging but possible long-term side effects of over-dosage have not been explored so at present routine high-dose vitamin D is not recommended for all women (Holmlund-Suila et al. JAMA Pediatr 2020. https://doi.org/10.1001/jamapedia trics.2019.6102).