Children born to parents who took more than a year to conceive or who used fertility treatment may be at increased risk of developing asthma, according to a new study.
While previous research has linked the use of assisted reproductive technologies, including IVF, to asthma risk, it is unclear whether this is due to unidentified factors associated with the parent's subfertility or the fertility treatments themselves.
To examine this, scientists from the Norwegian Institute of Public Health in Oslo studied more than 500,000 children, combining birth and prescription data from national Norwegian health registries and the Norwegian Mother and Child cohort study.
The data included information on fertility treatment, time to conception, and number and timing of any previous miscarriages. The scientists also considered factors such as the mothers' ages, weights before pregnancy and smoking habits during pregnancy.
For the study, childhood asthma was defined as the use of prescription asthma drugs in the preceding 12 months before the child turned seven.
The researchers found that just over four percent of children had asthma but demonstrated that children born via IVF were 42 percent more likely to develop asthma than children conceived naturally.
When these children were then compared with those whose parents had spontaneously conceived after more than 12 months, they were 22 percent more likely to have asthma. This suggests that the increased risk of asthma is not completely explained by factors related to parental subfertility, but likely influenced in some way by the procedure of ART, said the researchers.
They speculate that 'several steps in the assisted reproductive technology treatment may alter the natural course of the fetal development', proposing factors such as the drugs used to induce ovulation and maintain the pregnancy, the type of embryo culture medium, the freezing and thawing of fertilised embryos, and the hormonal environment.
The scientists also found that the number of previous miscarriages during early pregnancy was associated with an incrementally increased risk of asthma. They suggested that a common immune system mechanism may underlie the increased risk of asthma in children whose mothers had subfertility and miscarriages 'since immunological mechanisms contribute both to problems conceiving and repeat pregnancy losses'.
The data has received some tough scrutiny from experts however. 'The study did not have access to key confounders such as paternal smoking rates,' said Professor Alastair Sutcliffe at University College London. He also noted that 'the rates of "asthma" were implausibly low and this suggests a fault in how it was defined'.
Dr Jane Stewart, chair of British Fertility Society concluded: 'Whilst this study raises some interesting questions, there are many variables involved in both infertility and the development of asthma. Much more investigation would be required to establish any causal link.'
In an editorial accompanying the study in the journal Thorax, Dr Peter Gibson at the John Hunter Hospital in Newcastle, Australia agreed that the possible interactions leading to asthma risk are complex. He said that while the 'effect sizes are modest, their importance comes from the relatively high prevalence of both asthma and subfertility and the potential to modify these risk factors'.