Children whose mothers took antibiotics while they were pregnant were slightly more likely than other children to develop asthma, according to a Danish study.
The results don't prove that antibiotics caused the higher asthma risk, but they support a current theory that the body's own "friendly" bacteria have a role in whether a child develops asthma, and antibiotics can disrupt those beneficial bugs.
"We speculate that mothers' use of antibiotics changes the balance of natural bacteria, which is transmitted to the newborn, and that such unbalance bacteria in early life impact on the immune maturation in the newborn," said Hans Bisgaard, one of the study's authors and a professor at the University of Copenhagen.
Previous research has linked antibiotics taken during infancy to a higher risk of asthma, although some researchers have disputed those findings.
To look for effects starting at an even earlier point, Bisgaard and his colleagues gathered information from a Danish national birth database of more than 30,000 children born between 1997 and 2003, and followed for five years.
They found that about 7,300 of the children, or nearly one quarter, were exposed to antibiotics while their mothers were pregnant. Among them, just over three per cent, 238 children, were hospitalized for asthma by age five.
The study, which appeared in The Journal of Pediatrics, found that by contrast, about 2.5 per cent, or 581 of some 23,000 children whose mothers didn't take antibiotics, were hospitalized with asthma.
After taking into account other asthma risk factors, Bisgaard's team calculated that the children who had been exposed to antibiotics were 17 per cent more likely to be hospitalized for asthma.
Similarly, these children were also 18 per cent more likely to have been given a prescription for an asthma medication than children whose mothers did not take antibiotics when they were pregnant.
His team also looked at a smaller group of 411 children who were at higher risk for asthma because their mothers had the condition. They found that these children were twice as likely as their peers to develop asthma too if their mothers took antibiotics during the third trimester of pregnancy.
Others said that it was possible that something besides the antibiotics was responsible, such as the illness the drugs were prescribed for.
"This study, it doesn't tell us whether it's the antibiotic use or whether it's the infection. That's one thing we can't decipher," said Anita Kozryskyj, a professor at the University of Alberta who also studies the antibiotics-asthma link but wasn't involved in the new study.
The results don't suggest that women should avoid antibiotics since some infections can be quite dangerous to a foetus, she said, adding that Bisgaard's study suggests that the development of asthma might start before birth, something researchers hadn't studied very closely.
"We're beginning to appreciate that some of the origins of asthma and changes to the immune system, maybe they start earlier than right after birth," she added.