Extra vitamin D in pregnancy gave children stronger-looking bones at age 6. By 13, the difference had gone

Vitamin D helps the body take up calcium and phosphate, the minerals bone is built from. That has led to a hopeful idea: if a mother has plenty of vitamin D in late pregnancy, her child might lay down more bone early on, reach a higher 'peak bone mass' in their twenties, and be less likely to get osteoporosis decades later. Two large trials seemed to support the first step. Both found that children of mothers given extra vitamin D had denser bones in early childhood. The question nobody could answer yet was whether that head start lasts. A Danish team has now followed one of those trials to age 13, and published the results in The Lancet Regional Health - Europe on 25 September 2026.

The trial is part of COPSAC2010, a group of 623 Copenhagen-area families who joined during pregnancy in 2009 and 2010. From week 24 of pregnancy until a week after birth, half of the mothers took 2,400 IU of vitamin D3 a day and half a look-alike placebo, on top of the 400 IU a day that Danish health advice already recommends. Neither the families nor the researchers knew who was taking what. In practice it compared 2,800 IU with 400 IU a day (70 versus 10 micrograms). When the children were 3 and 6, whole-body bone scans showed more bone mineral in the high-dose group, as the team reported in 2020.

At 13, 416 of the 584 children born into the trial had another scan, using DXA, the same low-dose X-ray method used to check adults for osteoporosis. Total bone mineral content averaged 1,869 g in the high-dose group and 1,891 g in the standard-dose group. Once age, sex, height, weight and scanner were taken into account, the difference was 6.7 g, well within chance. Bone density was the same, and so were the results for boys and girls separately, for children born in the darker months (when the earlier effect had been strongest) and for children whose mothers started the trial low in vitamin D. Putting the scans from ages 3, 6 and 13 together also showed no overall difference. By 13, 99 children had broken a bone at least once: 53 fractures in the high-dose group and 61 in the standard-dose group, a difference that could easily be chance.

The authors read this as a real but short-lived boost that does not carry into adolescence, which, they write, does not support the idea that vitamin D in pregnancy alone shapes peak bone mass and later osteoporosis risk. There are caveats. The trial was designed to test asthma and wheezing, not bones, so it may have been too small to pick up modest differences. Three in ten children had no scan at 13, the fracture analysis was decided after the fact, and bone density is hard to read around the growth spurt, when bones lengthen faster than they fill in. The other large trial, MAVIDOS in the UK, has reported results up to age 6 or 7 so far; its later follow-up will show whether the same fade happens there.