A comprehensive Danish study reveals that high doses of vitamin D during pregnancy do not produce lasting improvements in children’s bone density or reduce fracture risk by their early teens, challenging long-held assumptions about prenatal supplementation’s benefits.
A large Danish trial has found that giving pregnant women high doses of vitamin D does not appear to strengthen their children’s bones by the time they reach early adolescence, even though the same children showed bone gains in earlier years.
The findings, published in The Lancet Regional Health – Europe, add a sobering note to a long-running idea in maternal health: that improving vitamin D status during pregnancy could help shape a child’s skeleton for life. Vitamin D helps the body absorb calcium and phosphate, both essential for bone formation, which is why researchers have spent years testing whether supplementation in pregnancy might raise peak bone mass and lower the risk of osteoporosis later on.
The study followed children from the Copenhagen Prospective Studies on Asthma in Childhood 2010 trial, known as COPSAC2010. Between March 2009 and November 2010, researchers enrolled 623 pregnant women in the Greater Copenhagen area at 24 weeks’ gestation and randomly assigned them to receive either a high daily dose of vitamin D3 or placebo. Both groups were also advised to take the Danish Health Authority’s standard 400 IU a day, meaning the comparison was effectively between 2,800 IU and 400 IU from mid-pregnancy until shortly after birth. Maternal vitamin D levels were checked with a laboratory method considered the gold standard, confirming that the supplement produced a biological effect.
Earlier reports from the same trial had raised expectations. At ages three and six, children whose mothers received the higher dose had shown higher whole-body bone mineral density and bone mineral content, especially those born in the darker winter months. Those results fitted neatly with the broader theory that prenatal vitamin D could create a lasting advantage, because bone mineral content in childhood is often linked with bone strength in adult life.
But the new 13-year follow-up did not support that hope. Of the original group, 416 children returned for full-body DXA scans, a standard imaging test used to measure bone mineral content and density. The researchers found no meaningful difference between the high-dose and standard-dose groups in total body bone mineral content, total body bone mineral density or the related total-body-less-head measures. Analyses by sex, puberty stage, maternal vitamin D status, birth season and participation in a separate fish-oil trial also failed to reveal any clear subgroup that benefited.
Fracture outcomes were similarly unconvincing. Among 550 children with complete follow-up, researchers recorded 114 radiologically verified fractures in 99 children, affecting bones such as the clavicle, radius, ulna, tibia, fibula, femur and humerus. Neither the time to first fracture nor the overall fracture rate differed significantly between the two groups.
The study did identify one possible clue: among children in the high-dose group, those with sufficient vitamin D levels at six months of age tended to have better bone measures at 13 than those who remained insufficient. The interaction between the prenatal supplement and early-life vitamin D status was statistically significant for total body bone mineral content, suggesting that what happens after birth may matter as much as what happens in the womb. Still, the authors treated that result cautiously, and the linked fracture analysis in that subgroup did not show a clear benefit.
The trial has important strengths. It was randomised, double-blinded and placebo-controlled, with long-term follow-up, radiologically verified fractures and repeated checks across different scanners and developmental stages. That makes it one of the strongest tests yet of whether prenatal vitamin D can deliver a lasting skeletal benefit. Even so, the authors note that the original study was designed around asthma outcomes rather than bone health, and that the adolescent scan results were incomplete for about a third of the children.
The Danish findings also sit alongside a growing body of newer research that points in the same direction. A post hoc analysis of the British MAVIDOS trial published in June found that extra vitamin D in pregnancy did not improve offspring bone microarchitecture. A separate trial in pregnant women with vitamin D deficiency, published in July, showed that higher doses could safely restore biochemical sufficiency in mothers and newborns but did not measurably change neonatal bone mineral content. Taken together, the studies suggest that prenatal vitamin D may be useful for correcting deficiency, but not a reliable lever for durable improvements in children’s bone strength.
Disclaimer: This content is for informational purposes only and is not intended to be a substitute for professional medical judgment, advice, diagnosis, or treatment.
