Editor's Note: Scientific literature generally uses the words "mother" and "maternal" in reference to a person with a uterus who has carried a child. To avoid inconsistencies, this article also uses that language.
To many, the mention of a chimera brings a vision of fantasy characters pieced together from other animals. But nature, too, creates chimeras — and nearly every human has, at one point, been one.
During pregnancy, fetal and maternal cells cross an area of the body called the placental barrier. In other words, for a period of time, there is an exchange of cells between mother and fetus. With that cellular trafficking, science shows that a portion of fetal cells stay in a maternal body after giving birth. They may exist there for months, years or even decades.
Some social media users point to this phenomenon to support messages against abortions. For instance, posts claim "fetal stem cells can remain in a mother's body for many years after pregnancy," making abortions at any point unnatural. One Instagram post equated fetal cells to a "child [who] is fighting to save your life," citing purported theories that the cells bring maternal health benefits.
It's true that cells involved in fetal creation remain in a mother's body post-birth. However, these cells that move through the placenta are not reflective of the fetus itself — a concept that the above-mentioned social media posts misconstrue. Depending on its development stage, a fetus comprises anywhere from one cell at conception to trillions of cells at birth, a portion of which were exchanged between mother and fetus.
Also, scientists do not fully understand the implications of these in-question cells. Any claims about their alleged health benefits are not widely supported by the scientific community. One-off studies have linked them to both improvements in maternal health, as well as harmful outcomes. Plus, the cells change over time and can be influenced by many factors.
Here's What's Happening as Fetuses Grow
As a fetus grows, cells are exchanged between the mother and fetus through the placenta, a disc of tissue that connects a mother's uterus to the umbilical cord. Nutrients, water, and oxygen move from the mother to the fetus unidirectionally.
Meanwhile, throughout the fetus' development, there is also bidirectional movement of cells and genetic material, including DNA. While those exchanges go both ways, there are often more fetal cells trafficked from the fetus to the maternal body, according to a 2022 review published in the journal iScience. Some fetal cells cross the placental barrier and enter the mother's blood, reaching organs throughout the body, including the brain, kidney, breast, heart, thyroid, and lymph nodes.
In humans, this exchange usually occurs between the fourth and sixth week of gestation and increases in the amount of cells trafficked throughout pregnancy. Then, in the first few weeks after birth, the mother's immune system eliminates most fetal cells and they gradually disappear from blood circulation.
But it's typical for some cells to stay with the mother.
Researchers first identified fetal cells in pregnant people in 1893. Nearly a century later, in the 1970s and 1980s, scientists found all pregnant people retain some forms of fetus cells, though how long they stay in maternal blood can vary from person to person. A study published in 1996 determined fetal cells can last in maternal blood for up to 27 years following delivery.
Put another way, scientists have found fetal cells throughout maternal tissues and organs during and after pregnancy. However, the amount of fetal cells that remain in a mother's bloodstream after birth varies.
As of this writing, it was not largely understood why fetal cells migrate and what happens after pregnancy to keep them in a person's system.
In other words, it was unknown why some people retain more fetal cells than other child-bearing people, as well as why the cells exist in some people's blood longer.
Theories Why Fetal Cells Stay in a Mother's Body
According to 2023 research by a group at Cincinnati Children's Hospital Medical Center, some cells transferred between a fetus to a mother's bloodstream are T cells, which the mother's body may keep long-term to "remember" prior pregnancies and assist in future ones.
Foreign entities that enter the body are normally attacked by the immune system, as is often the case in organ transplants. But a mother's immune system does not attack a growing fetus as if it was an invader. In part, that's because of the T cell exchange between mothers and fetuses, according to the 2023 research, which was conducted in mouse models.
But what about uteruses that are pregnant for the first time? According to the research, it's not just fetal cells from previous pregnancies that help prevent the immune system from attacking a growing fetus; the researchers found maternal microchimeric cells, or cells from a person's mother, may do the same thing. (In other words, the bloodstream of a first-time mother contains cells from her mother to do the job.) Also, for people who have given birth multiple times, the researchers found unique fetal microchimeric cells from each pregnancy.
"Very small numbers of fetal cells can be found in the heart, liver, intestine, uterus and other tissues [after birth]," wrote study author Sing Sing Way in a news release at the time. "The fact that we are made up of more than just cells with our own genetics, but also cells from our mothers and our children is a fascinating idea."
The Cells' Implications
Some scientists suggest the exchange of maternal-fetal cells improves fetal survival and maternal health — that is, that the presence of them after birth can help repair tissues and replace lost cells.
However, other researchers have noted that fetal chimerism may "play an essential role in maternal pathophysiology," or how diseases affect people who have been pregnant.
Some scientists suggest certain health issues appear in people who retain more fetal cells than others. In 2013, for example, researchers found women with preeclampsia retain fetal cells more often and at higher concentrations compared to women with uncomplicated pregnancies. Fetal microchimerism has also been linked to premature births and miscarriages and has been present in some cancer patients. Researchers also speculate fetal cells contribute to autoimmune disease.
Additionally, researchers have suggested the presence of fetal cells can modify the mother's health after delivery — potentially inducing changes in lactation, maternal affection, and neural plasticity (such changes may only benefit the baby, not the mother).
It's unknown why one person over another may be affected or may suffer issues as a result of retaining such cells.