Thousands of sickly newborns could be saved each year if officials closed some of the nation's smaller neonatal intensive care units, according to a new study that suggests larger hospitals are better able to treat the infants.

Extremely premature babies were up to twice as likely to survive when treated at a busy, advanced-care center instead of one of the many community hospitals that have opened ICUs in recent years.

Even among the most advanced centers, those that handled the most babies had the best survival records, said Ciaran Phibbs, lead author of the study appearing in Thursday's New England Journal of Medicine.

"Size really matters," said Phibbs, a Stanford University health economist.

Earlier studies found conflicting results when reviewing the relationship between neonatal deaths and number of infants treated by a hospital. But Phibbs' study is the largest and best of its kind, experts said.

"It's quite persuasive," said Dr. David Goodman, a Dartmouth Medical School neonatal care specialist who was not involved in the study.

The study reviewed nearly 48,000 premature births and fetal deaths in California from 1991 through 2000, using birth and death certificates and hospital records. Researchers focused on babies with very low birth weights of 1 to 3 pounds.

California's top neonatal intensive care units — called Level 3 NICUs — offer the full range of neonatal care and surgery. They had the best survival rates. Those that treated more than 100 premature babies each year had the lowest death rate — about 18 percent. In similar facilities that treated 50 to 100 infants a year, about 20 percent died.

The trend continued in less comprehensive neonatal units. The lower the level of care and the smaller the number of babies, the higher the death rate. In Level 2 NICUs that saw 10 or fewer tiny babies a year, more than 31 percent died.

The effect was seen across race and size differences in the infants studied, and in both genders.

Based on the results, researchers estimated that consolidating intensive care units could prevent 21 percent of deaths among especially small infants.

Why the difference? Hospitals with busy neonatal units also have advanced obstetrics care, including around-the-clock anesthesiology and other services to quickly handle emergencies, Phibbs said. They also have more practice.