Bed sharing between adults and young babies multiplies the child's risk of Sudden Infant Death Syndrome five times, even if the parent is a non-smoker and hasn't consumed drugs and alcohol.
The finding comes from a University of Auckland study, looking at adults who share a bed with their breastfed baby in the child's first three months.
While there is a general consensus that sleeping with a baby increases the risk of SIDS if the parents smoke or if the mother has been drinking alcohol or taken drugs, previously there have been conflicting opinions about whether bed sharing represented a risk when these factors were not present.
The researchers found the risk of SIDS was fivefold in comparison to when a baby slept in a cot in the parents' room, even without these other factors, said Professor Mitchell, said University of Auckland paediatrician Professor Ed Mitchell.
"Sudden Infant Death Syndrome (SIDS) is a major cause of death among babies aged under one in New Zealand and other high income countries.
"More than 50 per cent of deaths occur while parents slept in the same bed as baby," he said.
The study, the largest of its kind, involved researchers examining the individual records of 1472 SIDS cases and 4679 control cases across five major studies.
It also showed that the risk associated with bed sharing decreases as a baby gets older.
Risk of SIDS remained very low for babies whose mothers followed existing New Zealand advice: no smoking in pregnancy and around baby, sleep baby on the back, have baby in a cot in the parental bedroom and breastfeed if possible.
The Ministry of Health advises the safest place for your baby to sleep is in a cot beside the parents' bed for the first six months of life, Prof Mitchell said.
Some countries, including the United States and the Netherlands, advise all parents against sharing a bed with their baby for the first three months. The United Kingdom advises only certain groups, including parents who are smokers, not to bed share.
"Parents need to know the risks from bed sharing, especially for babies under three months.
"Health professionals have a duty to inform them. Innovative strategies such as the wahakura and pepi-pod provide alternatives to bed sharing, and are attracting interest overseas," said Professor Mitchell.
If parents were made more aware of the risks of sleeping with their baby, and instead room sharing was promoted "we could achieve a substantial reduction in SIDS rates", he said.
The authors said babies can still be brought into the parents' bed for comfort and feeding during the night, but should be placed in a cot next to the parents' bed to sleep.