Most experts recommend that pregnant women take a daily supplement of 400 units of vitamin D. Now a randomized clinical trial suggests that a much larger dose may be beneficial for children’s bone health.

The study, in JAMA Pediatrics, randomized 517 women to take either a 2,400-unit vitamin D supplement or a placebo from 24 weeks of pregnancy until one week after birth.

In addition, all the women were advised to take a 400-unit vitamin D supplement, in line with Danish health recommendations.

The researchers followed the offspring with periodic bone scans through age 6, recording bone density and evidence of fractures. Over all, children whose mothers took 2,800 units had significantly higher bone density at age 6 than children in the placebo group. The effects were particularly robust in women who were initially deficient in vitamin D and in those who gave birth during the winter, when sunlight levels are lower and vitamin D blood levels tend to drop.