Add one more strike against the unnecessary use of antibiotics: When used very early on in life, including during pregnancy, they seem to increase the risk of obesity later on. They appear to disrupt the gut’s natural array of “good” bacteria, which are known to play a role in our metabolic health and the risk for weight gain.

Farmers often give their animals antibiotics not only to reduce the risk of infection in the animals, but also to fatten them. The authors of a new study took their cue from the farming industry — since antibiotics seem to increase the rate at which an animal puts on weight, perhaps antibiotic exposure is a factor in weight gain more generally.

It seems that there’s a sensitive period right around birth, when the normal array of bacteria in an infant's digestive tract is just getting set up, that antibiotics are likely to do the most harm.

Researchers gave low-doses of penicillin to two groups of mice: Four-week old mouse pups and to pregnant mice right before they were due to give birth. They tracked their weight gain over time.

The mice that received penicillin in utero through their mothers were significantly more likely to be obese as they matured. This was especially true for male mice. In fact, just four weeks of exposure in these mice was enough to trigger metabolic changes and resulting weight gain.

Certain types of beneficial bacteria also were reduced when the young mice were given antibiotics.

It seems that there’s a sensitive period right around birth, when the normal array of bacteria in an infant's digestive tract is just getting set up, that antibiotics are likely to do the most harm. Infancy is a critical time during which the metabolism is especially vulnerable to bacterial disruption with antibiotics, explained study author Martin Blaser.

The findings support the idea that antibiotics should only be used in infants when clearly needed, for example, to fight an infection the child is unable to overcome on his or her own, he added.

Not only do antibiotics harm beneficial bacteria, but they’re known to have long-term health effects in humans, since they also damage the way our own cells operate.

In this new study, the authors found that the antibiotics were not hurting the cells of the mice, they were harming the bacteria that help the mice stay healthy and balanced.

One solution may be to prescribe probiotics as a way to counter the ill effects of antibiotics early in life.

“Our findings imply that restoring good bacteria could prevent the long-term metabolic effects of early antibiotic exposure,” lead author Laura Cox said in a statement. “We identified four candidate bacteria that may be metabolically protective, and we're working on follow-up studies to determine if we can prevent weight gain by giving these bacteria back following antibiotic therapy.”

If you’re pregnant and on antibiotics, or your young child is, talk with your doctor about ways to rebalance your system, since probiotics may or may not be appropriate. Eating fermented foods like yogurt, sauerkraut, and tempeh may be a simpler way to take in some good bacteria that will benefit the gut.

The study was carried out at New York University and published in the journal Cell.