Federal health officials say the parents of premature babies enrolled in a treatment study several years ago weren’t properly informed of potential risks.

Very premature babies need oxygen. Too much has long been known to cause blindness. Too little can increase risk of death. The study enrolled 1,300 babies at 23 hospitals between 2005 and 2009 to hunt the best dose somewhere in between.

In a March letter publicized Wednesday, the government’s Office for Human Research Protections said researchers didn’t spell out the risks to parents. The University of Alabama at Birmingham, which led the study, said although slightly more babies who received lower-dose oxygen died, the death rates were lower than usual for that time. But university officials pledged to better outline risks in future studies.