One shows that lying prone can twist a baby's neck so far that the arteries leading to the head become blocked, and another showed that nerve cells near brain arteries may not work properly.
Both support the recommendation that babies be put to sleep on their backs.
SIDS, also known as crib death, is a catch-all term used to describe inexplicable infant deaths. In the United States more than 2,000 babies die of SIDS every year.
This number used to be much higher. Following findings in 1992 that babies who slept on their bellies were at greater risk for SIDS, the American Academy of Pediatrics launched a campaign to promote back sleeping. Experts have credited it with a 40 percent drop in SIDS deaths.
A similar campaign had similar effects in Europe.
SIDS is also more likely in the babies of mothers who smoke, babies put down in overheated rooms and babies who are wrapped in blankets. A defect in liver enzymes is blamed in about 5 percent of cases and some research also points to a defect in the brain stem, where breathing is controlled.
Dr. Stefan Puig and colleagues at the University of Vienna in Austria looked at 12 babies, six who had died of SIDS, four who died of trauma such as a car accident and two who died of heart disease.
They twisted their heads the way a baby dies when it sleeps. "If you have a baby you know that they are much more flexible than adults," Puig said in a telephone interview.
Ultrasound was used to test the flow of blood in their neck arteries.
"We found that 71 percent of the children diagnosed with SIDS and 29 percent of the other infants had narrowed vertebral arteries when they were put on their stomach and their neck was rotated to the left or right," Puig said.
"When their neck was rotated and extended, half of both groups had compressed arteries," added Puig, who presented his findings to a meeting in Atlanta of the American Roentgen Ray Society. "This study suggests that narrowing of the vertebral artery could be a factor in SIDS.
A second study, done at Yale University, found that neurons thought to be important in SIDS are found near some of the largest arteries supplying the brain.
Dr. George Richerson and colleagues said the neurons produce serotonin, an important message-carrying chemical. Earlier research has shown that brain cells that hold serotonin are strongly stimulated by carbon dioxide -- suggesting that serotonin helps signal the brain that the gas is present.
"When someone falls asleep with their face in a pillow, carbon dioxide levels rise," Richersen said in a statement.
"The normal response is to wake up slightly, turn the head and breathe harder. There is evidence that some infants that die of SIDS lack this normal protective response.
If the neurons meant to detect dangerously high levels of carbon dioxide do not work, perhaps the baby suffocates instead of waking up and moving, Richersen said.
Puig stressed that SIDS is a broad description and that a range of causes are to blame for the mysterious deaths.
Last week Australian doctors reported that babies who died of SIDS had high levels in their blood of a toxin associated with E. coli bacteria
Other researchers recently reported that babies who are exposed to second-hand smoke have a higher risk of crib death.
By Maggie Fox