'It doesn't lie. So who are you?': What happens when DNA tests show a woman is not the mother of the child she gave birth to?

DNA is often considered the ultimate indicator of our identity — a foolproof way to determine our origins and how we connect to our parents and previous generations of our family. But in this excerpt from "Hidden Guests: Migrating Cells and How the New Science of Microchimerism Is Redefining Human Identity" (Greystone Books, 2025), author and science journalist Lise Barnéoud explores an unusual case that exposes the limitations of DNA testing, when a maternity test suggested a woman was not the mother of the children she gave birth to.

Lydia Fairchild was 26 years old when she applied for welfare benefits to help her raise her two children on her own. As part of the application process, she had to undergo a maternity test. A few weeks later, she was called into a meeting with social services, where they accused her of not being the mother of her children.

"At first, I kind of laughed … But they were serious. I could just see the seriousness in their faces," Fairchild said. "DNA is 100% foolproof, and it doesn't lie," a social worker told her. "So who are you?"

At first, Fairchild was suspected of attempting to defraud the welfare system by inventing children. The state prosecutor launched an investigation and quickly confirmed that two children did indeed live with her. Could she have kidnapped them? Fairchild showed them photographs of herself pregnant. Her mother, her children's father, and her obstetrician all testified to the fact that she had given birth.

Could she be a surrogate mother who kept the children she'd carried? After three hearings in court, Fairchild feared the worst. "Every day it felt like it was going to be the last day I'd see them," she tearfully recounted. "I called every lawyer in the phone book. None of them believed me. It was my word against DNA. It was me against everyone."

Fairchild was pregnant with her third child at the time, and the judge asked that both mother and child be tested immediately after birth. And the impossible happened: Fairchild's third child, just emerged from her womb, was not her son either — genetically speaking.

At last, a lawyer agreed to help her. Alan Tindell asked Fairchild about her life, her relationships with her siblings, and her relationship with the father of her children. "Given her answers, I finally decided to believe her," Tindell explained. He soon came across a scientific article describing Karen Keegan's case and contacted the team in Boston to ask them to examine Fairchild. They first tested Fairchild's blood, but they found only one cell type, just as they had for Karen Keegan. They moved on to cells from her skin, hair, and cheek: still nothing.

Until the day they performed a cervical smear. There, they found cells with a different DNA, a DNA that matched Fairchild's children as well as her mother. They concluded that the second DNA must have come from a vanished twin sister. Fairchild could finally breathe. But how would her story have ended without Karen Keegan?

The oft-taught equation of "one individual, one genome" fails to capture the full complexity of reality. What seemed a long-established and unshakable certainty, even to me, has turned out to be imperfect knowledge in need of revision. We know too little about our own biology to have blind faith that DNA profiling will always reveal a person's identity or origins.

Our ultimate proof is far from foolproof. Yet it is very often used to determine relationships, prove or disprove paternity, evaluate applications for family reunification, or convict persons otherwise presumed innocent. "The overriding assumption in such circumstances is that a sample that fails to confirm genetic kinship is an indication of fraud, regardless of other substantiations of legitimate kinship relations," observes the British philosopher Margrit Shildrick, one of the few scholars to examine the social and legal consequences of microchimerism.

Why is some scientific knowledge so hastily dressed up as infallible truth? Do we not dwell enough on our own ignorance? And why do some fields of knowledge remain frozen by skepticism, even when new discoveries should allow us to dispel our doubts? The sociology of science has its work cut out for it.

It's impossible to know how many Karen Keegans and Lydia Fairchilds exist. Most of the time, the existence of chimeric cells from vanished twins goes unnoticed. If Keegan had not needed a kidney transplant, if Fairchild had not applied for welfare benefits, they never would have known that their gametes were "occupied" by cells other than their own.

Their children or grandchildren might have eventually discovered that a branch in their family tree appeared to be missing, that they had somehow inherited genes that neither of their parents possessed.

Today, we know of about a dozen cases of this phenomenon, known as germ-line chimerism: where chimeric cells are present in the tissues that form eggs or sperm. One such case involved an American man who learned through a paternity test that he could not be the father of his child, who was conceived via assisted reproduction. He was preparing to sue the clinic, believing himself to be the victim of a semen mix-up, when a more precise test revealed that he in fact shared 25% of his DNA with the child. In other words, he was the child's uncle, genetically speaking.

Further research showed that 10% of his sperm contained DNA from a vanished twin brother. "One of the most impactful consequences of this case study is to point out that some traditional paternity tests which have resulted in negative outcomes (the tested parent was excluded as the biological parent) may have been wrong, because the alleged parent may have undiagnosed chimerism," stress the researchers who chronicled his case.

Given the increasing use of these tests, it is likely that the paternity of other fathers has been wrongly contested. This is precisely the scenario depicted in the French TV series "Nona et ses filles," which aired in 2021. Nona, played by 70-year-old actress Miou-Miou, is pregnant when a genetic test reveals that her lover, André, cannot be the father of her child. They eventually learn that one of André's testicles contains sperm from a vanished twin brother. In the words of André, as he attempts to parse his situation, "So he's my nephew … but he's also my son."