About 1 in 50 U.S. infants are victims of nonfatal child abuse or neglect in a year, according to the first national study of the problem in that age group.

The study focused on children younger than 1 year, and found nearly a third were one week old or younger when the abuse or neglect occurred.

"It is a particularly vulnerable group," said study co-author Rebecca Leeb, of the Centers for Disease Control and Prevention.

"We were struck by the fact there was a clustering of maltreatment with the very, very early age group."

The researchers counted more than 91,000 infant victims of abuse and neglect in the period Oct. 1, 2005 to Sept. 30, 2006.

The information came from a national data base of cases verified by protective services agencies in 45 states, the District of Columbia and Puerto Rico.

Other studies have looked at national child abuse and neglect cases, but this is believed to be the first to focus on infants, said Leeb, a CDC epidemiologist.

The 91,000 infants were age 1 year or younger. About 30,000 of those cases were infants aged one week or younger. About 68 percent of those cases were attributed to neglect.

Federal officials define neglect as a failure to meet a child's basic needs including housing, clothing, feeding and access to medical care. But the counted cases did not include new parents stumbling their way through breast-feeding or making other rookie mistakes.

"Things like abandonment and newborn drug addiction would qualify as neglect, not things like parents learning how to be parents," Leeb said.

Click here to comment on this story.

Medical professionals identified about 65 percent of the maltreated newborns to protective services staff. The others came from law enforcement, relatives, friends, neighbors and from protective services staff.

The results mirror what a study in Canada found, Leeb said.

The CDC collaborated on the study with the federal Administration for Children and Families.