FRIDAY, Jan. 4 (HealthDay News) -- Vitamin D deficiency early in pregnancy may increase the risk of having a baby with a lower birth weight, according to a new study.

Researchers looked at vitamin D levels in blood samples collected from more than 2,000 U.S. women who gave birth to full-term babies. Although the original collection was done between 1959 and 1965, the blood samples were well preserved.

Women with vitamin D levels of less than 0.015 parts per million in their first 26 weeks of pregnancy had babies who weighed an average of about 1.6 ounces less than normal, according to the study, which was published in the January issue of the Journal of Clinical Endocrinology & Metabolism.

In addition, women who were vitamin D deficient during their first 14 weeks of pregnancy were twice as likely to have babies whose weight was in the lowest 10 percent, which means they were small for gestational age.

Babies born small for gestational age have a higher risk of dying in their first month or developing chronic conditions such as heart disease, high blood pressure and type 2 diabetes later in life, said the researchers, from the University of Pittsburgh Graduate School of Public Health.

Vitamin D deficiency during pregnancy could cause low birth weight by hindering the typical increase in calcium absorption by the mother, which could reduce bone growth in the fetus, the study authors said.

Vitamin D deficiency could also cause a decrease in the hormones needed to produce the glucose and fatty acids that provide energy for the fetus.

"This is one of the largest studies to examine a mother's vitamin D levels and their relationship with birth weights," study senior author Lisa Bodnar said in a University of Pittsburgh news release. "It shows that clinical trials to determine if you can improve birth weights by giving women of reproductive age vitamin D supplements may be warranted."

Although the study suggested an association between pregnant women's vitamin D levels and babies' birth weights, it did not prove a cause-and-effect relationship.

More information

The American Academy of Family Physicians outlines how pregnant women can look after themselves and their developing baby.