Narrator: This is Science Today. For several reasons, premature babies tend to be anemic, which means they get lots of transfusions -- too many, says Dr. Rod Phibbs of the University of California, San Francisco. In a national study, Phibbs used a genetically engineered hormone called erythropoieten to reduce anemia, and thus transfusions, among preemies.

Phibbs: Well, this is something that we all make if we're reasonably healthy.

Narrator: In children and adults, erythropoieten is made in the kidneys. When the red blood cell count is low, a sensing system there sends erythropoieten into the bone marrow, where it stimulates red blood cell production.

Phibbs: The fetus uses erythropoieten to make red blood cells too, but the system in the fetus is in the liver. The erythropoieten producing system and the sensing system in the liver apparently turns off at birth, and the kidneys should turn on but they're not quite ready to.

Narrator: In Phibbs's study, injecting the hormone stimulated the babies' red blood cell production until their own erythropoieten systems kicked in -- reducing the need for transfusions. For Science Today, I'm Steve Tokar.