Children born to subfertile parents, who took more than a year to conceive or required fertility treatment, are more likely to develop asthma, say scientists.

Data from over 13,000 children, forming part of the UK Millennium Cohort Study, was analysed. Children born using assisted reproduction technologies (ART) were two to four times more likely to have asthma or have taken anti-asthmatic medication by age five than children planned and conceived naturally.

Lead researcher Dr Claire Carson, from the National Perinatal Epidemiology Unit at the University of Oxford, 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'.

In the study 15 percent of all children aged five had asthma. This figure rose to 24 percent among the 104 children born through ART, such as IVF and ICSI. In the UK, asthma is common in childhood, with around 20 percent of five-year-olds diagnosed with the condition.

The causes of asthma are not completely understood but both genetic and environmental factors play a role in its development. A link between childhood asthma and ART remained evident in the study after researchers controlled for confounding factors, including maternal asthma, family smoking, the presence of furry household pets and social class.

It is not known whether the increased presence of asthma in children born using ART is a result of the fertility treatment, the underlying infertility or another as yet unknown cause.

Malayka Rahman, research analysis and communications officer at the charity Asthma UK, cautioned that the study sample size was small and current research in this area is not conclusive: 'Further work is needed to establish what might be causing this association and whether there are other factors at play other than the IVF treatment itself. In the meantime those considering IVF should speak to their GP about the benefits and health risks in order to make an informed decision'.

The study was published in the journal Human Reproduction.