Pregnant women who get a little sunshine in their last trimester may help strengthen their unborn babies bones.

A U.K. study of nearly 7,000 kids found those who were born during the summer or sunnier months of the year generally had larger bones. Researchers theorize that vitamin D is the key, because sun exposure prompts the body to produce the vitamin.

Experts stop short of recommending pregnant women to spend time in the sun and say increasing vitamin D intake during pregnancy maybe beneficial.