A new study has shown that asthma was more common in children born after fertility treatments than those conceived naturally.

The study found that five-year-old children born to “sub-fertile parents” were more likely to have asthma, wheezing, and to be taking anti-asthmatic medication. “Sub-fertile” parents were define as those who tried for at least one year before successful conception or those who received a form of assisted reproduction technology (ART). The trend was greatest for children born after in vitro fertilisation (IVF) or intracytoplasmic sperm injection (ICSI), who were two to four times more likely to have asthma.

“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,” said Dr. Claire Carson, a researcher at the National Perinatal Epidemiology Unit at the University of Oxford. “Fifteen percent 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.

“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, published last month in the journal Human Reproduction, looked at 13,000 five-year-olds from the U.K. Millennium Cohort Study. Their mothers were surveyed about their conception and pregnancy, and the study was controlled for the mothers’ history of asthma, smoking, BMI, socioeconomic status, gestational age at the time of delivery, the type of delivery, and breast feeding.

The researchers stated that explanations for the link include the severity of infertility, over-reporting of asthmatic symptoms by overly protective parents, the infertility treatments themselves, or other factors not taken into account.

“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,” said Carson. “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.”