Women with low vitamin D levels during the first 26 weeks of pregnancy were more likely to have babies who weighed less than their peers, according to a study published in the Journal of Clinical Endocrinology & Metabolism. The researchers studied preserved blood samples of 2,146 pregnant women who participated in the Collaborative Perinatal Project from 1959 to 1965. Results would likely be different in a modern-day study, the researchers said.