British researchers say a woman's risk of developing a premature baby triples if she continues taking prenatal vitamins into the third trimester of pregnancy. They studied women from 2003 to 2008.
Doctors say it is possible that interactions between the vitamins and minerals led to a reduction in the nutrients available to the growing fetus.
Researchers say more studies are needed.
This report is in the British Journal of Obstetrics and Gynecology.
