Asthma is more common among children born after IVF and other treatments than among children who have been planned and conceived naturally, suggests a study led by Oxford University researchers.
However, the researchers say that their findings should not worry parents of children born after assisted reproduction technology (ART).
Dr Claire Carson, a member of the research team at Oxford University's National Perinatal Epidemiology Unit, said: 'Although the children born after ART were more likely to be diagnosed and treated for asthma than other children, it is important to remember that in absolute terms the difference is quite small.
'Fifteen per cent of the children in our study had asthma at the age of five. Although this figure was higher, 24%, in the IVF children, it isn't much higher than the one in five risk for all children in the UK.'
She added: 'Although we found an association, we cannot tell at this time if it is causal. Further research is needed to establish what might be causing the association and the underlying mechanism involved. It is also important to remember that for most children, asthma is a manageable condition and shouldn't prevent children from living a full and active life.'
The study found that at the age of five, children born to parents who had had to wait longer than a year before managing to conceive, or who had conceived via ART, were more likely to experience asthma, wheezing and to be taking anti-asthmatic medication.
The association was driven mainly by children born after in vitro fertilisation (IVF) or intracytoplasmic sperm injection (ICSI). These children were two to four times more likely to have asthma, wheezing or be taking anti-asthmatics.
The study by researchers at the universities of Oxford and Essex is published in the journal Human Reproduction.
The UK Millennium Cohort Study is following the progress of 18,818 children from across the UK who were born between 2000 and 2002. It is one of the few large observational studies where data are available on conception, asthma diagnoses and key social and lifestyle factors that could influence the results.
The Oxford and Essex researchers analysed data from surveys of the children at age five and seven, who were all single births (no twins were included).
The researchers asked mothers about conception and, once the children had reached the ages of five and seven, whether the children had ever had asthma, wheezing or were taking medications for treating asthma.
The researchers also took into account a range of other factors, including mothers' history of asthma, smoking, body mass index, socioeconomic status, any furry pets in household, and premature births.
Of the 18,818 children in the study, full data were available for 13,041 of the children at age five, and 11,585 by the time they were seven. Among the five year olds, 104 were born after ART.
The researchers say there could be a number of possible explanations for the association between infertility and asthma. These include: the severity of the infertility; infertility treatment; over-reporting of asthmatic symptoms by excessively protective ART parents (but the researchers think this is unlikely); or other, confounding factors that may not have been taken account of.
Dr Carson said: 'Childhood asthma is a common condition in the UK where the prevalence of the condition is higher than other European countries, and (to our knowledge) this is the first UK study of asthma after IVF conceptions.
'Our analysis suggests that it is the ART group in particular who are at higher risk. However, we do need to be reasonably cautious when interpreting the results because there is a relatively small number of IVF cases in our study – just 104 babies.'
The researchers, led by Maria Quigley of the National Perinatal Epidemiology Unit at Oxford University, will continue to follow the children's progress to see if the same effect can be seen at age 11. They say more research needs to be conducted in larger groups of children born after ART to see if their findings can be replicated.