WASHINGTON — Too often, newborns die of genetic diseases before doctors even know what’s to blame. Scientists, however, have found a way to decode those babies’ DNA in just days instead of weeks, moving gene-mapping closer to routine medical care.

The idea: Combine faster gene-analyzing machinery with new computer software that, at the push of a few buttons, uses a baby’s symptoms to zero in on the most suspicious mutations. The hope would be to start treatment earlier, or avoid futile care for lethal illnesses.

Wednesday’s study is a tentative first step: Researchers at Children’s Mercy Hospital in Kansas City, Mo., mapped the DNA of just five children, and the study wasn’t done in time to help most of them.

But the hospital finds the results promising enough that by year’s end, it plans to begin routine gene-mapping in its neonatal intensive care unit — and may offer testing for babies elsewhere, too — while further studies continue, said Dr. Stephen Kingsmore, director of the pediatric genome center at Children’s Mercy.

“For the first time, we can actually deliver genome information in time to make a difference,” predicted Kingsmore, whose team reported the method in the journal Science Translational Medicine.

Even if the diagnosis is a lethal disease, “the family will at least have an answer. They won’t have false hope,” he said.

More than 20 percent of infant deaths are due to a birth defect or genetic diseases, the kind caused by a problem with a single gene. While there are thousands of such diseases — from Tay-Sachs to the lesser known Pompe disease, standard newborn screening tests detect only a few of them. And once a baby shows symptoms, fast diagnosis becomes crucial.

Sequencing whole genomes — all of a person’s DNA — can help when it’s not clear what gene to suspect. But so far it has been used mainly for research, in part because it takes four to six weeks to complete and is expensive.