ATLANTA (Reuters) - Birth defects, including heart problems, rather than illnesses associated with premature births are a leading cause of death in very young babies in the United States, according to a report issued on Thursday.

Birth defects, such as a heart condition called pulmonary valve stenosis, are also the most common reason babies are hospitalized, said study researchers, from the University of Arkansas, who examined data on babies under 10 days old.

The study is the first to look nationally at the impact of birth defects and could be used to inform parents and caregivers about mortality rates for given conditions and likely costs associated with care, said James Robbins, professor of pediatrics at the University of Arkansas.

The study, reported by the U.S. Centers for Disease Control and Prevention, examined 35 birth defects using 2003 data from hospital admissions.

Around 3 percent to 4 percent of babies are born with a birth defect of some kind in the United States, he said, citing previous research.

The most common birth defects include hypospadias, a genital condition in boys, Down's syndrome, cleft lip and pulmonary valve stenosis.

Hypoplastic left heart syndrome, in which a baby is born with small or no left chambers of the heart, was the condition that led to the most expensive hospital care, Robbins said.

Some birth defects can be detected prior to birth through ultrasound but families of children born with birth defects are affected in myriad ways, including the high cost of medical treatment, and more research was needed on that, he said.

"We need to know more about how these conditions are affecting the families of the babies both in the neonatal period," as well as the subsequent financial, psychological and social burden, he said.