WASHINGTON — Too often, newborns die of genetic diseases before doctors even know what’s to blame. Now scientists 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 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, 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.

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, standard newborn screening tests detect only a few of them.

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

On Wednesday, researchers reported that the new process for whole-genome sequencing can take just 50 hours — half that time to perform the decoding from a drop of the baby’s blood, and the rest to analyze which of the DNA variations uncovered can explain the condition.

The study counted only the time the blood was being decoded or analyzed, not the days needed to ship the blood to Essex, England, home of a speedy new DNA decoding machine made by Illumina, Inc. — or to ship back the results for Children’s Mercy’s computer program to analyze.