In a new study published in JAMA Pediatrics, an international group of researchers conducted the first detailed look at pollution’s effect on developing babies in utero. They found that the more pollution expectant moms were exposed to while they were pregnant, the shorter their babies’ telomeres: parts of the DNA in every cell that act as a molecular clock keeping track of the cell’s age, and the body’s.

According to the study of 641 newborns, those whose mothers were exposed to higher levels of certain types of air pollution (so-called “particulate matter” from things like car emissions and burning of residential heating fuels), were born with shorter telomeres—8.8% shorter in their cord blood cells and 13.2% shorter in their placenta cells—than those whose mothers were exposed to less pollution. The effect was strongest when the moms were exposed during the second trimester.

Telomeres shorten every time a cell divides; since older cells have divided more than younger ones, their telomeres are shorter. Eventually, when the telomeres become too short, that signals the cell to die. The study’s findings suggest that these babies are starting out with a shallower reserve of telomere length—so as their cells divide, the cells will age faster than those that start out with longer telomeres.

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“Our results may have important health consequences later in life because a shorter telomere length at birth indicates less buffer capacity for postnatal influence of insults,” such as inflammation, the authors write. Other studies have shown that a number of things can affect the length of telomeres and accelerate their shortening, including smoking, obesity, exposure to violence and stress. All tend to increase inflammation, which promotes a molecular stress that quickens the telomere-shortening process.

Frederica Perera, director of the Columbia University Center for Children’s Environmental Health, has investigated the effects of pollutants on fetuses and says the study’s findings are noteworthy. “The findings are yet another argument for addressing the problem of air pollution,” she says. “Shortened telomere length is certainly not a good sign.” (Perera was not involved in the new research.)

The researchers say their study does not suggest that all children born in environments where their mothers were exposed to pollution will necessarily age more quickly. They point to some encouraging news in their finding that babies born to mothers exposed to higher levels of air pollution during their third trimester tended to have longer telomeres than those exposed to less. This suggests that by the final trimester, the fetus may have developed ways to compensate for and counteract the effects of pollution on the DNA, they say.

The authors say that these results should promote more research into the effects of particulates on developing cells in utero. Longer term studies of how the shortened telomeres affect children and teens are also critical. (Perera, for example, is tracking telomere length in a group of children, now 18 and 19, whose in utero exposure was measured.) If pollution can affect aging in adult cells—and research suggests it does—then more work needs to be done to better understand how it can influence aging in newborns as well.