"Size really matters," said Phibbs, a Stanford University health economist.
Earlier studies had found conflicting results when reviewing the relationship between neonatal deaths and number of infants treated by a hospital.
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.
California's top neonatal intensive care units, which offer the full range of neonatal care and surgery, had the best survival rates. Those that treated more than 100 premature babies each year had the lowest death rate, about 18 percent, compared with 20 percent in similar facilities that treated 50 to 100 infants annually.
The trend continued in less comprehensive neonatal units. The lower the level of care and the fewer the babies, the higher the death rate. The effect was seen across race and size differences in the infants studied, and in both genders.
Researchers estimated that consolidating ICUs could prevent 21 percent of deaths among especially smaller infants.
Why the difference? Hospitals with busy neonatal units also have advanced obstetrics care, including services to handle emergencies, Phibbs said. They also have more practice.
Debby Rogers, vice president of quality and emergency services for the California Hospital Association, said closing NICUs carries risk. 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.