"If your clinical group takes care of a lot of patients year after year, you start to identify trends you would miss with less volume. Experience lends knowledge and wisdom, to not just physicians but the entire team," said Dr. Aaron Caughey, a study co-author from the University of California at San Francisco.

Phibbs said the issue is apparent in metropolitan areas such as Stockton, Calif., which has 100 to 150 extremely premature births each year. Most of those cases are distributed among three hospital NICUs, all within a few miles of each other.

"It's certainly feasible there, if they worked out the agreement, for all those high-risk deliveries to be done in one place," he said.

Historically, the expertise and technology needed to run an NICU have been scarce, and care was concentrated at academic medical centers. In some states, laws or regulations confined services to a few hospitals.

But in the last 20 years, the number of NICUs boomed — from 578 in 1985 to 838 in 2004, one industry survey found. In California, the number rose from 156 in 1991 to 168 in 2000.

One reason is more doctors specializing in neonatal care, Goodman said. Hospitals have also recognized that such care can be lucrative. Virtually all NICU patients have public or private insurance, and hospitals charge a lot for this kind of care. "NICUs make money for hospitals," Caughey said.

Operating an NICU is a source of pride — and a focus of marketing — for hospitals interested in attracting obstetrics business, experts said.

Marketing materials are some of the only information expectant parents have available when choosing a birthing center, because hospital-specific infant mortality data is not available to the public.

"They may never know they're potentially putting their kids at risk" by choosing hospitals with lower-volume, less-comprehensive neonatal care, said Dr. Kevin Schulman, director of the health sector management program at Duke University's business school.

Hospital industry officials acknowledged that such information is not available, but new Web sites such as www.hospitalcompare.hhs.gov and www.calhospitalcompare.org may help.

Debby Rogers, vice president of quality and emergency services for the California Hospital Association, said closing NICUs carries some risk.

Residents of some rural areas have limited access to advanced medical services, and it's better for them to have limited NICU care than no such care at all, she said.

Phibbs said his team considered that argument, and found 92 percent of the births in 2000 occurred in urban areas with more than 100 such deliveries each year.