Children born with the help of fertility treatments are more likely to develop asthma than those conceived naturally, a University of Oxford-led study suggests.

By the time they're five, children born after in vitro fertilization or intracytoplasmic sperm injection are two to four times more likely to have asthma, experience wheezing or be taking anti-asthmatics, the study of more than 18,000 U.K. children found.

But Dr. Clare Carson, one of the researchers, said it's no cause for panic for parents considering fertility treatment.

She noted that "in absolute terms, the difference is quite small."

"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," she said. "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."