The American Academy of Pediatrics recommends that infants sleep on their backs to reduce the risk of SIDS and was involved in a 1990s "Back-to-Sleep" campaign that helped reduce the nationwide SIDS rate by more than 40 percent.
While stomach sleeping decreased during that time, SIDS still kills nearly 3,000 infants each year - and premature babies face a disproportionate risk.
Some parents and doctors may mistakenly believe "that the `back to sleep' message perhaps doesn't apply to low-birthweight or preterm infants," said the lead researcher, Dr. Louis Vernacchio of Boston University.
The study found that stomach-sleeping among very small premature babies has declined. Still, in 1998, 17.5 percent of mothers reported putting very small preemies to sleep on their stomachs one month after leaving the hospital, compared with 12.8 percent of mothers of the largest babies studied.
Rates of stomach sleeping increased at three months, which is also when rates of SIDS peak.
The study appears in the March edition of Pediatrics, published Monday. Doctors from the National Institutes of Health also were involved in the report.
The researchers surveyed mothers of 907 low-birthweight babies in Massachusetts and Ohio from 1995 to 1998. Vernacchio said similar results likely would be found nationwide. Babies' birth weights ranged from less than 3.4 pounds to about 5 1/2 pounds.
The overall rate of infant stomach-sleeping reported by mothers dropped from 20 percent to 11.4 percent during the study. Among the smallest babies, it dropped from 34 percent to 17.5 percent.
Parents who placed very small premature infants on their stomachs often said that their babies and their doctors seemed to prefer that sleep position, the study found.
Vernacchio said the risks of SIDS outweigh any medical problem that might prompt a doctor to recommend stomach-sleeping for premature babies.
University of Virginia pediatrician Dr. John Kattwinkel said many doctors who care for very small premature babies prefer stomach-sleeping while infants are in the intensive care unit, where they are constantly monitored. Such infants often have lung problems, and doctors think they can breathe easier on their stomachs, Kattwinkel said.
Doctors should - but often don't - switch premature babies to back-sleeping toward the end of their hospital stays, said Kattwinkel, who chairs an American Academy of Pediatrics task force on SIDS.
Parents who see their hospitalized babies sleeping on their stomachs may assume that's the correct position at home, Kattwinkel said.