Listeria monocytogenes can cause miscarriage, stillbirth and premature labor in pregnant women.

Listeria, a common food-borne bacterium, may pose a greater risk of miscarriage in the early stages of pregnancy than appreciated, according to researchers at the University of Wisconsin-Madison School of Veterinary Medicine studying how pathogens affect fetal development and change the outcome of pregnancy.

“For many years, listeria has been associated with adverse outcomes in pregnancy, but particularly at the end of pregnancy,” says Ted Golos, a UW-Madison reproductive physiologist and professor of comparative biosciences and obstetrics and gynecology. “What wasn’t known with much clarity before this study is that it appears it’s a severe risk factor in early pregnancy.”

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Pregnant women are warned to avoid many of the foods — among them unpasteurized milk and soft cheese, raw sprouts, melon and deli meats not carefully handled — that can harbor listeria, because the bacterium is known to cause miscarriage and stillbirth, and spur premature labor. Those severe outcomes have resulted in a zero-tolerance regulatory policy for listeria in ready-to-eat foods. But when it occurs, listeria infection in pregnancy may go unnoticed. The few recognizable symptoms are nearly indistinguishable from the discomfort most newly pregnant women feel.

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