Youths whose mothers took 2,800 units of vitamin D daily during pregnancy had significantly increased bone density at age 6 and a lower rate of bone fractures, compared with those whose mothers took a 400-unit vitamin D supplement during pregnancy, according to a study in JAMA Pediatrics. Researchers also found that elevated prenatal vitamin D exposure was most strongly linked to improved childhood bone density among those whose mothers had vitamin D deficiency and those who were born in the winter, but didn't affect birth weight or height and weight at age 6.