On the second Sunday in May, Mother’s Day celebrates the women who care for us, the maternal bonds between mother and child, and the influence of mothers in society. Mothers come in all forms and what they instill in their children goes far beyond the childrens’ genetics. The founder of Mother’s Day in the US, Anna Jarvis, famously believed that a mother is “the person who has done more for you than anyone in the world.” On a genetic level, our biological mothers contribute half of our DNA at conception, and our biological fathers the second half. This is followed by 9 months of pregnancy as the embryo develops into a fetus and ultimately, a baby. But the genetic bond between mother and child does not end at conception. Did you know that both mom and baby also share each other’s cells during pregnancy? As early as the second week of pregnancy, there is a two-way flow of cells and DNA between the fetus and the mother. Cells containing DNA from the fetus cross the placenta and enter the mother’s blood circulation, while cells from the mother cross in the opposite direction and transfer into fetal circulation. Most of the cells coming from the fetus are destroyed by the mom’s immune system, but some persist and become embedded in various organs, and become a part of the parent. This phenomenon is called microchimerism, from the word “chimera,” referring to a mythical creature made from parts of different animals. How long does microchimerism last? The fetus typically transfers more of their cells to the mother than the other way around. This exchange begins as early as the first few weeks of pregnancy. The exchange between mom and baby has been shown in other mammals like dogs, cows, mice, and other relatives, suggesting that this cell exchange has occurred for approximately 93 million years. The fetal cells have been found to stay in the mother’s body beyond the time of pregnancy, and in some cases for as long as decades after the birth of the baby. The mom’s cells also stay in the baby’s blood and tissues for decades, including in organs like the pancreas, heart, and skin. In one study, more than half of adults still had maternal cells in their blood. In some cases, even cells from maternal grandmothers – acquired during a mother’s own gestation – can be transferred to the fetus. Because some fetal cells stay in the mom’s body for years, they are also sometimes transferred to future brothers and sisters of the first child. In this way, older siblings can contribute their cells to those of their younger siblings. The fate of the traveling cells What happens to these fetal cells once they reach mom’s body? Some studies show that fetal cells are beneficial to moms, and help in healing maternal wounds – interestingly, in mice, fetal cells actively journey to the site of injury. Furthermore, these fetal cells have been found in the healed scars following caesarian section and actively participate in the healing process by producing various skin components, like collagen. The fetal cells in the mother can also transform into cells needed by the mother, including brain cells, heart cells, and various cells of the immune system. Fetal cells are also found in other organs where it is speculated that they may enhance the future survival of the fetus. For example, fetal cells in the breast may play a role in increasing milk supply for the baby after birth. Other studies show a connection between the presence of these cells in the body of the mother and certain cancers or autoimmune diseases. For example, individuals with type 1 diabetes mellitus are more likely to have their mothers’ cells in their pancreas. However, it is too soon to jump to any conclusions about these findings — researchers are still trying to understand whether these cells could be a part of the cause or just innocent bystanders. Another theory is that the maternal cells found in the pancreas are actually helping with healing and regeneration of the organ. Regardless of the reasons behind it, what is certain is that mothers and their children carry a piece of each other for many years following pregnancy and birth. References Boddy AM, Fortunato A, Wilson sayres M, Aktipis A. Fetal microchimerism and maternal health: a review andevolutionary analysis of cooperation and conflict beyond the womb. Bioessays. 2015;37(10):1106-18. Gammill HS, Harrington WE. Microchimerism: Defining and redefining the prepregnancy context – A review. Placenta. 2017;60:130-133. Mahmood U, O’donoghue K. Microchimeric fetal cells play a role in maternal wound healing after pregnancy. Chimerism. 2014;5(2):40-52. Thamban, T., Agarwaal, V., Basu, A., Rajeev, R., Sinha, A., Dwivedi, A. P., & Khosla, S. (2019). Epigenetic inheritance across multiple generations. Transgenerational Epigenetics, 401–420. doi:10.1016/b978-0-12-816363-4.00019-5 Vanzyl B, Planas R, Ye Y, et al. Why are levels of maternal microchimerism higher in type 1 diabetes pancreas?. Chimerism. 2010;1(2):45-50. Ye J, Vives-pi M, Gillespie KM. Maternal microchimerism: increased in the insulin positive compartment of type 1 diabetes pancreas but not in infiltrating immune cells or replicating islet cells. PLoS ONE. 2014;9(1):e86985.