ATLANTA - Birth defects lead to more than $2.5 billion a year in hospital costs alone, according to the first national studies to estimate their financial burden on US families.

The reports are the most comprehensive to look at the costs of birth defects in at least 10 years, experts said.

They also will give many families their first real idea of the expense of a baby born with serious birth defects, said Dr. Nancy Green, medical director for the March of Dimes, which campaigns for birth defects prevention.

"People who are pregnant don't want to think about adverse outcomes, but these are unhappy realities," Green said. "Getting the word out about their presence and impact helps people understand better about these risks."

The risk of having a child with a birth defect is 3 to 4 out of every 100 babies born, according to March of Dimes officials.

The first study, released Thursday, was done by researchers at the University of Arkansas and the US Centers for Disease Control and used 2003 data from 36 states.

They looked at what hospitals charged patients - not the actual cost of care, which is just a fraction of hospital charges. They did not include physicians' bills or other medical charges related to birth defect care.

The researchers looked at cases in which a child had one of 35 obvious and serious birth defects, and was under 10 days old at the time the baby was admitted to a hospital for care for that defect.

Certain birth defects were particularly deadly: For example, about 85 percent of babies born with anencephaly - that is, born without all or most of their brain and skull - died in the hospital. Most died within two days of birth.

Because of their brief life span and the limited options for care, the average hospital bill for one of these cases was $3,800.

The longest hospital stays were for children with surgically repaired gastroschisis, a defect involving an opening in the abdomen through which intestines stick out. The condition kept children in hospitals for 41 days, on average. The average bill was one of the highest among birth defects - about $156,000.

The most expensive condition was hypoplastic left heart, in which an infant is almost or completely missing the two left chambers of the heart. Treatment is a heart transplant or a series of reconstructive surgeries, and the condition required a 29-day stay in the hospital that cost about $200,000, on average.

"It (the bill) is almost always a function of how long they were in the hospital because of the surgeries that were done," said James Robbins, the Arkansas researcher who led the study.

Some defects can be detected during pregnancy through tests like amniocentesis and ultrasound, including anencephaly, spina bifida and chromosomal abnormalities. But some heart problems are more subtle and go unrecognized until after birth, Robbins said.

Experts say pre-birth diagnosis can help families decide to search for an obstetrician or hospital specializing in high-risk births, or make financial and other plans to deal with the diagnosis.

A second study released this week, by the US Agency for Healthcare Research and Quality, looked at 2004 data from 37 states.

Those researchers measured something different. They estimated what it cost hospitals to care for birth defects, which they reasoned was about 40 percent of what the hospitals charged.

And they looked not only at newborns, but at a sample of people of all ages who had hospital stays primarily for the treatment of birth defects.

They found the average age of patients was about 17 1/2 years, the average hospital stay about six days, and the average per-stay cost was $18,600. The aggregated cost for all these hospital visits was more than $2.5 billion, the researchers found.

"These (birth defects) are expensive - many of them are surgically treated - and the impact on society becomes large when you add this up," Green said.

The March of Dimes is planning to petition the US Food and Drug Administration to increase requirements that food be enriched with folic acid, which lowers the risk for spina bifida and certain other birth defects.