Giving pregnant women vitamin D could mean their babies grow stronger bones in later life, a study suggests.
Children born to the women in the study were tested nine years later
A study of 198 mothers indicated the children of those who lacked the vitamin, crucial for calcium absorption, had weaker bones at nine.
Those who took supplements or were exposed to more sunlight, which helps the body grow its own vitamin D, had children with greater bone densities.
The research from Southampton General Hospital is published in the Lancet.
Professor Cyrus Cooper, who led the team, said the findings provided evidence that maternal vitamin D status during pregnancy influenced the bone growth of offspring and their risk of osteoporosis in later life.
He told the BBC News it was the vitamin deficiency of the mother carrying the child, rather than the baby in early life, which affected the child's bone strength later.
"This is completely new - no one has ever looked at the mother's vitamin D levels before."
Vitamin D is crucial for the absorption of calcium which is in itself key in the formation of healthy bones.
The team from the Medical Research Council's Epidemiological Resource Centre at Southampton General Hospital measured the levels of vitamin D in women's blood in late pregnancy as well as studying calcium levels in the babies' cord blood.
This showed how vitamin D had helped calcium transfer across the placenta.
Nine years after the babies' delivery, the team traced 198 of the original 596 mothers who remained in the Southampton area and measured their children's bone mineral content and bone mineral density.
Professor Cooper now wants to carry out a study to see whether supplementation of vitamin D deficient pregnant mothers could lead to stronger bones in their babies in later life.
Professor James Walker of the Royal College of Obstetricians and Gynaecologists said the study demonstrated the importance of having adequate levels of vitamin D in pregnancy, both for the mother and her baby.
But he said it demonstrated that women who had adequate vitamin D levels were fine, and it was "only when levels were deficient that there was a problem".
"More vitamin D is not necessarily good," he said. "Therefore, no woman should take extra vitamin D in pregnancy unless recommended by their doctor."
Jackie Parrington, spokeswoman for the National Osteoporosis Society, said the research was important as it showed the need to look after one's bones started at an earlier age than had previously been thought.
"Maintaining bone health is important throughout life. Regular weight bearing exercise and a healthy balanced diet are all essential for keeping our skeletons strong as are stopping smoking and not drinking heavily," she said.
Osteoporosis, which costs the NHS £5 million a day, affects half of women and one in five men over 50 in the UK. It results in bones becoming so porous that they can break very easily.