Children whose mothers took antibiotics while they were pregnant were slightly more likely than other kids to develop asthma in a new 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 unbalanced bacteria in early life impact on the immune maturation in the newborn," said Dr. Hans Bisgaard, one of the authors of the study and a professor at the University of Copenhagen.
Those effects on the immune system could lead to asthma later on, although it's still not clear how, said Anita Kozyrskyj, a professor at the University of Alberta who also studies the antibiotics-asthma link but wasn't involved in the new study.
Previous research has linked antibiotics taken during infancy to a higher risk of asthma, although some researchers have disputed those findings (see Reuters Health stories of May 17, 2011 and February 3, 2011).
To look for effects starting at an even earlier point in a baby's development, 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 percent (238 kids) were hospitalized for asthma by age five.
In comparison, about 2.5 percent, or 581 of some 23,000 kids whose mothers didn't take antibiotics were hospitalized for asthma.
After taking into account other asthma risk factors, Bisgaard's team calculated that the children who had been exposed to antibiotics were 17 percent more likely to be hospitalized for asthma.
Similarly, these children were also 18 percent more likely to have been given a prescription for an asthma medication than kids whose mothers did not take antibiotics when they were pregnant, according to findings published in The Journal of Pediatrics.
In an email to Reuters Health, Bisgaard said he expected to see a higher risk of asthma "because the mother is a prime source of early bacterial colonization of the child, and antibiotics may (have) disturbed her normal bacterial flora."
Bisgaard's team also looked at a smaller group of 411 kids who were at higher risk for asthma because their mothers had the condition and found these children were twice as likely as their peers to develop asthma too if their mothers took antibiotics during the third trimester of pregnancy.
Kozyrskyj, who is research chair of the Women and Children's Health Research Institute, said it's also possible that something other than the antibiotics are to blame for the findings in both groups of children - such as the illness that caused the mothers to take antibiotics.
"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," she told Reuters Health.
The results don't suggest that women should avoid taking antibiotics to try to reduce their kids' risk of asthma, Kozyrskyj emphasized.
Some infections can be quite dangerous to a fetus, and "there are very good indications for these antibiotics," she added.
Bisgaard agreed that women should be treated, "but we see 1/3 of pregnant women in our region receiving treatments (often for urinary tract infections), which may reflect an uncritical use," he wrote in an email.
Bisgaard said his group is also studying the types of bacteria in pregnant mothers and newborn children to get a better understanding of their role in asthma.
Kozyrskyj said 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. It might be happening in utero," she said.