She said the recommendations covered a year “to be on the prudent side,” but that the first six months were most important because that’s when the risk of SIDS is highest.

“We don’t know why, but it appears to be protective to have baby sleeping in the same room as parents, not in the same bed but in the same room,” Dr. Hauck said.

To back this up, the academy cited three studies and an out-of-print book, “Sudden Infant Death Syndrome (SIDS): New Trends in the Nineties.” This is the same evidence the academy cited in 2011, when it also recommended room-sharing, but less prominently.

The three studies are case-control studies. That means researchers compiled records of babies who died of SIDS and matched them as closely as possible, based on things like age or geography. Then, by comparing the groups, they tried to identify factors that might place babies at higher risk.

Case-control studies are among the weaker designs used in human research. They can only indicate association, not causation, and they are especially susceptible to “recall bias,” when people remember things differently based on outcomes. People who have had a baby die, for instance, are more likely to have pored over any details that might have contributed.

Unfortunately, when it comes to SIDS, case-control studies are pretty much our only option. Even though SIDS is a large cause of infant mortality, very few babies die of it each year. We would never be able to enroll enough babies in a controlled trial to test interventions.

The first of the three studies was published in 1999 in The British Medical Journal. It matched 325 cases of SIDS against 1,300 controls, or babies who did not die of SIDS, in England. The second study, published in The Lancet in 2004, included 745 cases and 2,411 controls from 20 regions in Europe. The third, published in The Journal of Pediatrics in 2005, compared 123 cases to 263 controls in Scotland. All three found, to varying degrees, that infants who slept in a separate room had a significantly higher rate of SIDS than those who shared a room with their parents.