Her Blood Carried Her Twin Brother’s DNA, and Doctors Found It Only After a Miscarriage

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Yet she had normal female anatomy, normal hormones, and regular periods.

Further testing in Brazil revealed the explanation: her blood cells did not genetically belong to her. They matched her fraternal twin brother. The case, published in the journal Gene Reports in 2025, is an unusually complete example of blood chimerism, in which a person carries two genetically distinct cell populations.

The investigation was led by geneticist Caio Robledo D’Angioli Costa Quaio of Hospital Israelita Albert Einstein in São Paulo, with colleagues including Dr. Gustavo Arantes Rosa Maciel, a gynecologist and professor at the University of São Paulo’s medical school.

According to a conference abstract describing the case, the woman was 35 when she lost a pregnancy at about seven weeks. Doctors ordered a blood karyotype, a picture of a person’s chromosomes. Two separate blood karyotypes came back 46,XY in every cell examined, and a technique called FISH showed one X and one Y signal in all 100 cells analyzed.

She had reached puberty normally and had regular cycles, with no remarkable health events in her life.

Her other tissues told a different story. Cells from her urine, the inside of her cheek, and a skin biopsy were all 46,XX, the typical female pattern.

DNA fingerprinting with short tandem repeat markers and immune-system HLA typing showed her blood profile was identical to her twin brother’s, including the sex markers, and different from the rest of her body.

The likely explanation lies in the womb. In some twin pregnancies, the placentas share blood vessel connections, allowing blood-forming stem cells to cross from one fetus to the other. If those cells settle permanently in the bone marrow, a person can produce a twin’s blood for life.

“There was a process called feto-fetal transfusion,” Maciel told the French science outlet Futura-Sciences.

In her case, the takeover appears to have been complete in the blood, while the cells that formed her ovaries, skin and other tissues remained her own.

Her fertility was not affected. About 11 months after the miscarriage, she conceived again and gave birth to a healthy boy. Maciel has said the blood circulating in her body does not interfere with her eggs, so her children inherit only her own genetic material and her husband’s.

Small amounts of mixed blood are not unusual among twins. A 1996 study using a highly sensitive fluorescence method found blood group chimerism in 32 of 415 twin pairs, or 8%, and in 12 of 57 triplet sets, or 21%, according to Vrije Universiteit Amsterdam. Most of those cases involved small fractions of foreign cells, far from the complete replacement seen in Brazil.

Near-complete blood chimerism has been reported before. A 2021 report in Frontiers in Genetics described a Chinese woman undergoing infertility care whose blood was about 93% her twin brother’s, while no chimerism was detected in her cheek lining or the lining of her uterus.

Researchers have linked many such cases to fraternal twins who shared a single placenta, an arrangement once considered almost impossible. A 2017 systematic review of 31 such cases in Twin Research and Human Genetics found that assisted reproduction accounted for 82.1% of those pregnancies, and that the degree of blood chimerism ranged from 1% to 99%.

Most people with blood chimerism never find out. The Brazilian authors believe it is probably underdiagnosed because chromosome testing is rarely done on healthy adults.

Prof. Idit Maya, a medical geneticist at Rabin Medical Center in Israel who was not involved in the case, told The Media Line that the woman “had nothing outside or inside to say that her blood cells are from her brother.”

That matters because many genetic tests, from prenatal screening to diagnostic panels, rely on blood samples. In a chimeric person, a blood test may describe the twin’s genes rather than the patient’s own. Clinicians who see results that do not fit the patient, such as an XY karyotype in a woman with typical female development, may need to test a second tissue such as cheek cells or skin.

Maya also noted the case could affect organ transplant decisions, since the woman’s immune profile in blood matches her brother’s.

People who receive unexpected genetic test results should discuss them with a genetic counselor or medical geneticist before drawing conclusions.

It is a condition in which a person’s blood cells come from a different genetic source, most often a twin, while other body tissues carry the person’s own DNA.

How did the woman’s blood become her brother’s?

Researchers believe blood-forming cells crossed between the twins through shared placental blood vessels before birth and settled in her bone marrow.

No. She had normal female development and hormones, and she later gave birth to a healthy son who carries her own genes.

Through a blood chromosome test ordered after an early miscarriage, followed by testing of urine, cheek, and skin cells.

Why does chimerism matter for medical testing?

Blood-based genetic tests may reflect a twin’s genes. Unexpected results may require testing another tissue to get an accurate picture.