Vitamin D deficiency during pregnancy may be linked to a softening of bones in a baby's skull, Japanese researchers report.

They also found that breast-feeding without vitamin D supplementation could prolong the deficiency, which might lead to a risk of serious health problems later in life, including decreased bone density and type 1 diabetes.

"Craniotabes, the softening of skull bones, in otherwise normal newborns has largely been regarded as a physiological condition without the need for treatment. Our findings, however, show that this untreated condition may be the result of a potentially dangerous vitamin D deficiency," Dr. Tohru Yorifuji, of Kyoto University Hospital, said in a prepared statement.

The researchers evaluated 1,120 newborns and found that, at five to seven days of age, 246 (22 percent) of the babies had craniotabes. The researchers also found strong seasonal variations in the incidence of craniotabes. This suggests that the condition is associated with prenatal vitamin D deficiency, and is likely influenced by the amount of sunlight exposure a woman gets during pregnancy. The body produces vitamin D when the skin is exposed to sunlight.

This vitamin D deficiency in newborns may persist into later life, especially in breast-fed infants who don't receive a formula with vitamin D supplementation, the researchers said. More than half the breast-fed infants with craniotabes showed statistically significant low levels of serum 25-OH vitamin D, the storage form of the vitamin. Some of these infants had symptoms of an overactive parathyroid gland, which is also consistent with vitamin D deficiency.

The study appears in the Journal of Clinical Endocrinology & Metabolism.

"Until more research is done on the effects of perinatal vitamin D deficiency, we suggest treating breast-fed infants with craniotabes with vitamin D, or preferably, treating all pregnant women with vitamin D," Yorifuji said.

Several recent studies have reported a resurgence of vitamin D deficiency, even in developed countries, according to background information in a news release about the study. Skeletal problems, such as rickets in childhood or softening of bones in adults, are common consequences of vitamin D deficiency, which can also increase the risk of multiple sclerosis, type 1 diabetes and colorectal cancer in adults.