In the 1990s, scientists found the first clues that cells from both sons and daughters can escape from the uterus and spread through a mother’s body. They called the phenomenon fetal microchimerism, after the chimera, a monster from Greek mythology that was part lion, goat and dragon.

But fetal cells don’t just drift passively. Studies of female mice show that fetal cells that end up in their hearts develop into cardiac tissue. “They’re becoming beating heart cells,” said Dr. J. Lee Nelson, an expert on microchimerism at the Fred Hutchinson Cancer Research Center in Seattle.

The new study suggests that women almost always acquire fetal cells each time they are pregnant. They have been detected as early as seven weeks into a pregnancy. In later years, the cells may disappear, but sometimes, the cells settle in for a lifetime. In a 2012 study, Dr. Nelson and her colleagues examined the brains of 59 deceased older women and found Y chromosomes in 63 percent of them. (Many studies on fetal microchimerism focus on the cells left behind by sons, because they are easier to distinguish from the cells of their mother.)

Experts now believe that microchimerism is far from rare. “Most of us think that it’s very common, if not universal,” Dr. Nelson said. But it remains quite mysterious.

In recent years, researchers have found many clues suggesting that microchimerism can affect a woman’s health. Tumors may be loaded with fetal cells, for example, suggesting that they might help drive cancer. Yet other studies have suggested that fetal microchimerism protects women against the disease.