Fertility drugs can more than double the chances of children born to mothers who struggle to get pregnant developing leukaemia, a study has shown.
Children were 2.6 times more likely to become ill with acute lymphoblastic leukaemia (ALL), the most common type of childhood leukaemia, if their mothers had been treated with ovary-stimulating drugs.
They had a 2.3-fold increased risk of suffering the rarer form of the disease, acute myeloid leukaemia (AML).
Children conceived naturally after their mothers waited more than a year to get pregnant had a 50% greater-than-normal likelihood of developing ALL.
But no heightened risk of childhood leukaemia was associated either with in-vitro fertilisation (IVF) or artificial insemination.
The French scientists cannot yet fully explain their findings, the first to show a specific link between use of fertility drugs and childhood leukaemia.
Study leader Dr Jeremie Rudant, from the Centre for Research in Epidemiology and Population Health at the French research institute INSERM in Villejuif, Paris, said: "It has always been hypothesised that assisted reproductive technologies may be involved in the onset of childhood cancer as they involve repeated treatment at the time of conception and or manipulation of the sperm and egg. And it is now established that a majority of acute leukaemia have a pre-natal (pre-birth) origin.
"The findings indicate that more research is now needed to investigate more closely the link between specific types of fertility drugs and what role the underlying causes of infertility may play in the potential development of childhood leukaemia."
Dr Rudant presented the results at the Childhood Cancer 2012 conference in London, hosted by the charity Children with Cancer UK.
A total of 2,445 French children and their mothers took part in the study, comprising 764 children who had been diagnosed with leukaemia and 1,681 who were free of the disease. Mothers were asked if they had taken more than a year to conceive a child, and questioned about the treatments they had received.
Around 44,000 cycles of fertility treatment are carried out each year in the UK.
Use of fertility technology is increasing worldwide. In the UK, 1.8% of all live births in 2007 followed fertility treatment, compared with just 0.5% in 1992.
Despite a significant increase in risk, the actual number of children developing leukaemia after their mothers undergo fertility treatment remains very small.
Just 400 cases of childhood leukaemia are diagnosed each year in the UK, three-quarters of which are ALL.
ALL can affect children of any age but is most common between the ages of one and four. It is also more likely to affect boys than girls.
Dr Rudant said: "Previous studies have suggested a link between infertility treatments and acute childhood leukaemia but there haven't been many studies, most of them have been small and they focused either on IVF or hormonal treatment. Our study was much larger and it's the first time that a specific increased risk linked to fertility drugs has been found."