Babies who are bottle-fed in bed are at a greater risk of being smothered than those who are breastfed, researchers have claimed.
The study looked at breast and bottle-feeding mothers
They say this is because a mother who is breastfeeding naturally adopts a safe position, curled around the baby with its head at breast level and her knees under its feet.
But mothers who are bottle-feeding are much more likely to turn their backs on their baby, or put its head at the same level as their own, say the researchers from Durham University.
Mothers often take their babies into bed if they need a feed in the night.
We might be able to instruct non-breastfeeding mothers to sleep in this safer manner
Emma Kitching, University of Durham
Up to two thirds of parents are believed to sleep with their babies in the first three months.
But experts have been divided over whether bed-sharing increases the risk of smothering.
University of Durham researchers studied 40 couples and their babies.
They were filmed at night in the university's sleep lab under infra-red lights or at home.
It was found that breast-feeding mothers adopted the safest position automatically, but bottle-feeding mothers did not.
Emma Kitching, project co-ordinator of the research, told BBC News Online: "Breast-feeding mothers are more aware of the baby and keep them in this protective environment.
"They are not on the pillow, so there is no risk of them being suffocated by it.
"The position also means father's cannot roll onto the baby, and neither can the mother, because her arms are in the way."
She added: "We might be able to instruct non-breastfeeding mothers to sleep in this safer manner, but we need further research to see if that is possible."
The research also found that babies sleeping with their parents were 0.1°C warmer than those who slept in cots.
Although sharing a bed had been thought to make babies overheat, the Durham researchers found the slight increase was not likely to cause problems.
They said it was simply a result of the baby spending more time in REM (rapid eye movement) sleep, rather than in quiet or deep sleep.
Parents are not advised to share a bed with their babies if they have been smoking, drinking or taking drugs because they could be more likely to roll over onto them by accident.
The research was presented to a meeting held by the Foundation for the Study of Infant Deaths.