Pesticides and other pollutants in the environment may contribute to childhood leukaemia, say UK scientists.
Pesticides crossed the placenta
In laboratory studies the Bristol University team showed pollutants were able to travel across the placenta to the unborn baby.
The scientists presented their findings to a conference held by the Children with Leukaemia charity in London.
Experts doubted pesticides were involved in most cases and said more evidence was needed.
Leukaemia is the term used to describe a number of cancers of the blood cells.
In children, about 85% of these are acute lymphoblastic leukaemia or ALL and acute myeloid leukaemia (AML) accounts for most of the rest.
Dr Margaret Sanders and colleagues carried out tests on donated human placentas and pregnant guinea pigs.
They found compounds used as pesticides, such as DDT and other organochlorine compounds, readily crossed the placenta.
Starts in the womb
In the guinea pigs, the compounds accumulated in different organs of the unborn baby.
Co-researcher Professor Alan Preece said: "What the findings show is that if the mother takes these [pesticide compounds] in in food, they do get through to the foetus.
"The exact levels are as yet unknown but we know that childhood leukaemia starts in the womb and this could well be a factor."
He said more research was needed to find out how this might contribute to leukaemia.
"All we can assume is that, like everything else that a foetus is exposed to, like ionising radiation, they are more sensitive because the cells are all developing and turning over rapidly."
But he said this was only one of many factors that might be important in childhood leukaemia.
Professor Mel Greaves from the Institute of Cancer Research in London said: "We do not think pesticides are important for the common form of leukaemia in children.
"We have found a possible link with a rare form and we published a study three years ago. This was in young babies.
"For older children there is virtually no evidence that pesticides are a factor."
He said there was a study underway in California looking at the possible link between pesticides and childhood leukaemia.
He thought this might provide greater understanding when it reports over the next couple of years.
Professor Greaves said available evidence pointed to genetic factors and exposure to infections during childhood as potentially important contributors.
The Leukaemia Research Fund called for more research.