BOSTON (AP) — A breakthrough in the care of premature babies is proving to be a bargain as well as a life saver, in sharp contrast to other new treatments that are blamed for driving up medical costs.

A new study credits the development of an artificial substance that helps undersize babies breathe with bringing about a nationwide drop in infant mortality four years ago. Furthermore, the new treatment saves roughly $90 million a year by reducing the severity of newborns’ health problems.

The treatment means these babies require less time on respirators and other expensive treatments, said Rachael Schwartz, an epidemiologist who directed the study.

“As a result, their charges are lower, even though the length of stay did not go down,” she said.

Respiratory distress syndrome is the leading cause of death in premature babies. The reason is the immaturity of their lungs. They lack enough surfactant, a chemical that keeps open the tiny air sacs in their lungs by reducing surface tension.

Late in 1989, two brands of replacement surfactant became available — Exosurf, made by Burroughs Wellcome Co. and Survanta, made by Ross Laboratories.

The surfactant comes as a foamy liquid that is delivered directly into the lungs. Cost of treatment is estimated at $500 to 1,000 per baby.

Almost immediately, the infant death rate began to fall.