Toddlers who climb into bed with their parents have an increased risk of asthma in later childhood, a study has found.
The same association was not seen in bed-sharing infants, say researchers who were unable to provide a clear explanation for their findings.
One theory, that parents who notice wheezing symptoms in their children are more likely to keep them close at night, was not confirmed by analysis.
The study of 6160 mothers and their children in Rotterdam, the Netherlands, showed no link between babies sleeping with parents and an increased risk of wheezing or asthma in the first six years of life.
But bed-sharing at age two led to a 42 per cent higher chance of wheezing symptoms at age three to six, and a 57 per cent greater likelihood of being diagnosed with asthma at age six.
Lead researcher Dr Maartje Luijk, of Erasmus University in Rotterdam, said: "We postulated that the finding may be explained by parents taking the decision to share a bed with their toddler to monitor their asthma symptoms. However, our results found no associations between pre-existing asthma symptoms in the first two years of life and bed-sharing at the age of two years.
"This could suggest that bed-sharing increases the risk of asthma in some way, but this study does not provide causal evidence of this."
Dr Maartje Luijk said a number of factors could be at play. For example, bed-sharing families might be more likely to report wheezing because they are more attentive or aware of their children's breathing.
Alternatively, families might perceive wheezing as problematic and as something that could lead to sleep problems, which might elicit bed-sharing to better monitor these problems.
"More research is needed to identify the factors that may impact on the development of asthma through bed-sharing," Dr Luijk said.
The findings are reported in the European Respiratory Journal.
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