Children who are breast-fed as babies tend to have straighter teeth than those fed from a bottle, a Johns Hopkins study reports. Misaligned teeth were found 40 percent more often in children who were bottle-fed, or breast-fed less than one year, than in children who were breast-fed more than a year, the study found.

Based on mothers' reports of the dental history of 9,698 children, the study found that children who breast-fed for three months or less were among those with the highest incidence of crooked teeth.

Breast-feeding may protect against misaligned teeth by causing "a different growth pattern in the mouth," said Dr. Miriam H. Labbok, assistant professor of population dynamics at the Hopkins School of Public Health, who conducted the study along with researchers from the National Center for Health Statistics.

Infants who breast-feed use their tongues differently from bottle-feeders, Labbok said.

As a baby sucks, the tongue sinks back in the mouth to draw in the milk, she explained. With bottle-feeding, the tongue must move forward to stop milk coming from the nipple during swallowing. Breast-feeding requires no forward tongue thrust, so infants must use mouth muscles more vigorously.