A pregnant woman does many things to keep her growing baby healthy, from changing her eating habits to exercising. Now researchers are starting to discover that the baby has a long-lasting effect on its mother’s health, too.

Fetal cells cross through the placenta, moving into the mother’s body during pregnancy. In cases of pregnant women who had appendectomies, cells from their yet-to-be-born children turned up in the removed appendix, potential evidence that the cells are affecting the tissue. Once the baby is born, these cells can remain with the mother for decades. Fetal cells may be able to form new tissues and even help repair organs, says Dr. J. Lee Nelson, an autoimmunity researcher and rheumatologist at Fred Hutchinson Cancer Research Center.

These cells might also affect a woman’s susceptibility to disease. Nelson has been studying how fetal cells might affect a mother’s susceptibility to autoimmune diseases. Already known is the fact that during pregnancy, women with rheumatoid arthritis (RA) often notice symptoms vanishing. Nelson and her colleagues also found that mothers were almost 40 percent less likely to get RA than childless women. And cells passed from baby-to-be to mother might play a protective role in other diseases, too. In several studies, researchers have demonstrated that women who carry fetal DNA—which is found by looking for the male Y chromosome (simply because it’s easier to pinpoint than fetal female X chromosomes)—have a lower risk of breast cancer than those who don’t bear children.

Sometimes, though, cells from baby-to-be can make a woman more susceptible to certain types of disease. One is scleroderma, which affects connective tissue, with fetal cells being found in the skin and tissue of women who suffer from it. In a 2012 study, researchers also found that women with the highest levels of fetal cells in their circulating blood were four times more likely to develop colon cancer. The result was a surprise, says Dr. V. K. Gadi, the study’s senior author and a medical oncologist at Fred Hutchinson Cancer Research Center, and points to the difference in breast and colon cancer development, acknowledging that foreign cells could be driving an inflammatory response that contributes to colon cancer. 

Now researchers are interested in finding out exactly how fetal cells affect a mother in order to develop new therapies—making use of the cells’ protective qualities or blocking the harmful attributes they bring. And, says Gadi, if a woman’s fetal-cell level were factored into her breast cancer risk calculation, physicians could be able to know who might benefit the most from diagnostic preventive tests.

All of which means our cells aren’t just our own, but a connection to our past, our future, and our health. Or, as Nelson puts it: “We should really begin with a different concept of what the biological self is.”