It’s often been said that becoming a mother means you’ll carry a bit of your baby in your heart forever. It turns out that’s kind of true, biologically speaking: Research shows genetic material from babies remains in their mothers’ bodies for years after birth.

This transfer of cells from a genetically distinct feature to the mother is called fetal microchimerism. According to researcher Dr. Amy Boddy, it happens during every single pregnancy, even those that end in miscarriage. The cells have also been found to still be present in mothers’ bodies for decades after they’ve given birth.

“We currently do not know if or how fetal cells impact the mother's health during or after pregnancy,” says Dr. Boddy. “These cells may simply be by-products of the pregnancy and have no impact on the mother's body.”

The theories, however, are pretty interesting. As Boddy explains, one leading thought is that the cells migrate through the placenta to mom in order to help sustain the baby after birth through lactation and bonding.

“We have proposed an idea that these fetal cells may be useful in transferring resources to the baby after pregnancy. This is a theoretical framework based on evolutionary biology,” Boddy says. “Imagine that the fetus is setting up a sort of insurance policy within the mother to make sure there are resources after birth.”

In other words, the fetus sends out cells into the mom’s body to ensure she keeps protecting it after birth.

It follows that there may be a conflict over resources, with the fetal cells pulling for the baby, not the host, long after the baby has moved out of the womb—depending on the tissue where the fetal microchimerism occurs.

“If there were to be conflict it might be in tissues important in resource transfer to the baby,” Boddy explains, noting that such areas include the breast tissue, the thyroid (as it’s important for heat transfers), the brain (for attachment and bonding) or in the blood (if the mother's immune system attacks the microchimeric cells).

According to Body, researchers are very interested in how a baby’s cells manage to stay in a mom’s body for decades after a pregnancy, as the answer could further research into organ transplantation and autoimmune diseases.

Some studies suggest that that fetal cells, which are stem-like cells, may contribute to wound healing in the mother's body or protect against Alzheimer's. Other work indicates they play a role in the development of rheumatoid arthritis. Understanding fetal microchimerism could also be beneficial for treating breastfeeding problems and maybe even psychological disorders associated with pregnancy.

There’s a ton of work still yet to be done, but the current body of research suggests our kids’ leftover cells lead paradoxical lives inside our bodies; maybe giving, maybe taking or maybe just coming along for the ride.

Amid all the questions we still have about fetal microchimerism it is comforting to know that we’ll always carry a bit of our children with us. Even though mouse and human studies show the highest frequency of fetal cells are detected in the lungs, not the heart, we say that’s close enough.