Microchimerism Explained: How Pregnancy, Twins and Transplants Can Leave Foreign Cells in Your Body

More than a century ago, German pathologist Georg Schmorl identified a remarkable phenomenon: fetal cells that had crossed into a mother’s body during pregnancy. His observation was an early description of microchimerism — a biological process in which cells from one person migrate into another person and remain there.

Modern technology has allowed scientists to study microchimerism in far greater detail. Researchers now know that cells can move between a fetus and its mother, between twins, across generations and even between bone-marrow transplant recipients and their donors.

These discoveries are changing how scientists understand pregnancy, the immune system, genetics, DNA testing and human identity.

Cells transferred during pregnancy could mean that a person carries cells from their grandmother. In some cases, a vanished twin may leave cells behind in the surviving sibling. After a bone marrow transplant, donor cells can also travel beyond the recipient’s blood and appear in other tissues, including semen. In one documented case, this complicated a criminal investigation and raised questions about the reliability of DNA evidence.

These examples show why microchimerism is important. The phenomenon offers new insights into immunology, genetics, reproduction, tissue repair and evolution.

In her book Hidden Guests: Migrating Cells and How the New Science of Microchimerism Is Redefining Human Identity (Greystone Books, 2025), science journalist Lise Barnéoud examines how scientists discovered microchimerism and what these long-lived “guest” cells may do inside the body.

Hidden Guests has been shortlisted for the 2026 Royal Society Trivedi Science Book Prize.

What is microchimerism?

Excerpt from Hidden Guests: “It doesn’t lie. So who are you?” What happens when DNA tests suggest that a woman is not the biological mother of the child she gave birth to?

Live Science spoke with Barnéoud about the science of microchimerism, the movement of cells during pregnancy and the potential medical applications of these hidden cells. Hidden Guests has been shortlisted for the 2026 Royal Society Trivedi Science Book Prize.

How did you become interested in microchimerism?

Hannah Osborne: What first attracted you to microchimerism, and how did your understanding of the subject change while writing Hidden Guests?

Lise Barnéoud: I first encountered the subject while writing a previous book about vaccines and the immune system. I wanted to explain that the traditional image of immunity as an army that attacks every foreign substance is incomplete.

Pregnancy is an obvious example. A fetus is genetically different from its mother, yet the mother’s body generally tolerates the pregnancy. The microbiome provides another example: our bodies contain vast communities of bacterial, viral and fungal cells, many of which are essential to our health.

Microchimerism was a third example of how the body can coexist with cells that are not genetically identical to it. The more I learned, the more the subject challenged familiar ideas about the placenta, inheritance and immune defense.

The placenta is not an entirely sealed barrier. Cells can cross between a mother and fetus in both directions. A mother can retain cells from her children, while a child may carry cells that originated in the mother — or potentially from earlier generations.

At first, I imagined microchimeric cells as passive passengers. I later realized that they can be biologically active. They may divide, produce proteins, communicate with nearby cells and affect the body in ways that could be helpful or harmful.

Why did microchimerism research take so long to develop?

Hannah Osborne: Scientists first observed cells from one person inside another more than 100 years ago. Why did it take so long for microchimerism to become a major area of research?

Lise Barnéoud: The first observation dates to 1893, when a German pathologist identified fetal-origin cells in the lungs of women who had died during pregnancy. However, it took nearly a century to understand that cell exchange during pregnancy was common and potentially significant.

One obstacle was technology. Researchers needed tools capable of locating and studying very small populations of foreign cells within tissue. New molecular and imaging techniques have made that work increasingly possible.

Another challenge involved the traditional view of the immune system. If immunity is understood only as a defensive system that eliminates foreign material, it is difficult to explain how genetically different cells can remain in the body for years or even decades.

The field also faced an unfortunate cultural barrier. Some early researchers — many of them women — were told that microchimerism was merely an extension of maternal bonding or “women’s biology.” That was a serious misunderstanding. Men are affected by microchimerism as well, because every person develops in a womb and can receive or retain cells from other people.

Today, research is more diverse and the field is growing. Because microchimerism is relatively new, many fundamental questions remain open.

How can microchimerism affect DNA tests?

Hannah Osborne: Your book includes several cases in which microchimerism affected people’s lives directly. Which story stayed with you most strongly?

Lise Barnéoud: The case of Lydia Fairchild is especially striking because a DNA test nearly caused her to lose custody of her children.

Fairchild was applying for welfare assistance in the United States and was required to provide DNA evidence showing that she was the mother of her children. The test indicated that she could not be their biological mother. She had photographs and witnesses proving that she had given birth, but officials continued to treat the DNA result as conclusive.

She was pregnant with her third child at the time, so investigators tested the baby immediately after birth. The result again appeared impossible: the child’s DNA did not match Fairchild’s.

Further investigation revealed that Fairchild was a chimera. Early in development, cells from a vanished twin had merged with her body. Some of the cells that contributed to her reproductive organs carried her twin’s genetic signature. As a result, the DNA in her saliva and skin did not necessarily match the DNA in the eggs that produced her children.

This case demonstrates that DNA is extraordinarily powerful evidence, but it is not always as simple or absolute as people assume. The sample, the tissue tested and the individual’s biology all matter.

If microchimeric cells are present only in very small numbers, they are unlikely to affect a standard DNA test. However, in unusual cases, foreign cells may form a substantial proportion of a particular tissue or organ. The earlier in development the cells arrive, the greater their potential influence may be.

Vanishing twins may be more common than many people realize. Some studies suggest that a significant number of pregnancies begin with more than one embryo, although it is difficult to determine how often one embryo disappears before it can be detected through medical imaging.

It is probably uncommon for a vanished twin’s cells to alter a DNA test completely. Nevertheless, the possibility is important in unusual medical, genetic and forensic cases.

Microchimerism and forensic DNA evidence

Another case occurred in Alaska in 2004. Investigators examining a sexual-assault case found semen containing DNA that matched a man in a criminal database. However, that man was in prison when the assault occurred and could not have committed the crime.

The explanation was a bone marrow transplant. The man in the database had received bone marrow from his brother. Some of his brother’s cells had entered tissues outside the bone marrow and blood, including the reproductive system. The DNA in the semen therefore matched the donor rather than the recipient.

In this case, the recipient’s prison record prevented investigators from pursuing the wrong suspect. Without that information, the DNA evidence could have led to a false accusation.

Microchimerism does not make DNA testing unreliable in general. It does show that forensic scientists must understand which tissue produced a sample and whether a person has undergone a transplant or has another condition that could create genetically different cell populations.

More research is needed to determine how frequently microchimerism affects forensic evidence and how testing protocols should account for it.

Why is microchimerism open to misinterpretation?

Hannah Osborne: Microchimerism has been used in arguments about abortion, relationships and identity. Why does the subject invite so many different interpretations?

Lise Barnéoud: Microchimerism has a strong emotional impact because it connects biology with family, pregnancy, loss and identity. People are fascinated by the idea that cells from a mother, child or vanished twin may remain inside the body.

That emotional response can encourage people to attach meanings to the science that have not been demonstrated. Some political commentators have used fetal microchimerism to make claims about abortion, while others have suggested that these cells could be used to reverse aging or regenerate organs.

The scientific evidence is more nuanced. Microchimeric cells may influence health, but researchers are still determining when they are beneficial, when they may contribute to disease and how long they remain active. It is important not to confuse a plausible hypothesis with an established medical fact.

Microchimerism is a young field. Scientists are expected to propose hypotheses and test them, but the public should distinguish between confirmed findings, preliminary research and speculation.

Can a baby inherit cells from its grandmother?

Hannah Osborne: Which microchimerism hypothesis do you find most intriguing?

Lise Barnéoud: One surprising possibility is that a child may carry cells that originated not only from its mother but also from its grandmother.

A mother can retain cells from her own mother. During pregnancy, some of those cells may potentially cross into the developing fetus. For many years, scientists considered this idea unlikely. More recent research has provided evidence that newborns can carry cells originating from a grandparent.

This form of multigenerational microchimerism could help explain how cells move through family lines without being passed on as DNA. Genetic inheritance involves chromosomes, while cellular inheritance may involve living cells that migrate between generations.

Another proposed source is a sexual partner. Semen contains sperm, but it also contains immune cells and other cell types. Scientists are investigating whether some of these cells could enter a partner’s body and persist there. At present, this remains an area of research rather than a proven explanation for human microchimerism.

Researchers should be willing to investigate unusual ideas, provided they test them rigorously. At the same time, scientific uncertainty should not be used to present speculation as fact.

Could microchimerism help repair damaged tissue?

Hannah Osborne: What could be the most promising medical application of microchimerism during the next 10 to 20 years?

Lise Barnéoud: Tissue regeneration is one of the most promising areas. Researchers have found that microchimeric cells may participate in tissue repair. Clinical studies are investigating whether these cells could help repair damage after a heart attack, stroke or skin injury.

Future treatments might encourage a person’s existing microchimeric cells to move toward damaged tissue. Another possibility is that specially selected cells could be introduced therapeutically. However, these approaches remain experimental and require extensive testing for safety and effectiveness.

Microchimerism may also have applications in organ transplantation. Transplant recipients usually need lifelong immunosuppressive medication to prevent rejection. These drugs can have serious side effects. If scientists learn how microchimeric cells promote immune tolerance, they may eventually develop ways to reduce the need for immunosuppression.

Understanding how foreign cells coexist with the immune system could therefore lead to new treatments for tissue repair, transplantation and autoimmune disease. For now, scientists are still working to determine exactly how these cells behave and which medical uses are realistic.

Learn more about microchimerism