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Researchers used whole-genome sequencing to reconstruct a rare case in which a soft-tissue cancer began in one identical twin and crossed to the other before birth. The study authors say the tumour spread only once. This is evidence of an unusual transfer in a shared prenatal environment—not evidence that cancer is generally contagious.
How did cancer pass between twins before birth?
In a 2026 Nature Communications study, researchers sequenced multiple tumour, normal, and placental samples to trace the tumour’s development and relationships among the twins’ cells. The resulting genetic reconstruction indicated that the soft-tissue tumour originated in one twin, diversified, and then crossed to the other twin in utero. The authors report a single transfer between the twins.
The work describes three distinct stages: the tumour’s origin in one twin, its prenatal transfer, and its subsequent development. Whole-genome sequencing let the team compare genetic changes across the sampled tissues and reconstruct their lineage. The abstract also says the approach provided insight into early twinning and embryonic lineage contributions. It does not provide enough detail to establish the exact timing of transfer, sample counts, or the tumour subtype.
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This case shows that cancer cells can, in rare circumstances, move between twins before birth. It does not mean cancer spreads like an ordinary infection. The report concerns a particular tumour in a shared fetal and placental context; it does not establish that prenatal transfer is common or that cancer generally passes between people through contact.
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In an identical-twin pregnancy, shared development and placental blood-vessel connections can provide a route for cells to pass between fetuses. That is a different circumstance from everyday exposure to another person. The new study is a reconstruction of one unusual case, not a measure of how often such transfer happens.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How this case compares with earlier twin leukemia research
Earlier research on childhood leukemia provides a related example of suspected prenatal transfer, but the disease and evidence differ from the new soft-tissue tumour case.
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| Evidence | What researchers reported |
|---|---|
| 2026 soft-tissue cancer study | Whole-genome sequencing of tumour, normal, and placental samples supported a reconstruction in which a tumour began in one monozygotic twin and crossed to the other once. |
| 2003 twin leukemia report | Two-year-old monozygotic twins had concordant hyperdiploid B-cell precursor acute lymphoblastic leukemia (ALL) and shared clonotypic sequence evidence. The authors concluded that leukemia likely began prenatally in one fetus and spread through connections between placental blood vessels. |
| 2004 twin leukemia case report | Neonatal blood spots from twins had an identical TEL-AML1 fusion sequence but distinct immunoglobulin gene rearrangements, supporting the presence of separate preleukemic clones before birth. |
In the leukemia reports, a shared initiating clone did not necessarily mean identical later disease. Researchers describe independent genetic changes after the initial event as one possible explanation for why one twin may develop overt leukemia while the other remains clinically unaffected. That leukemia evidence helps explain how prenatal cell sharing can coexist with different outcomes; it should not be treated as proof that every cancer behaves the same way.
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What the findings do—and do not—show
- They show: Researchers reconstructed one prenatal transfer of a soft-tissue tumour between monozygotic twins using genetic evidence from tumour, normal, and placental samples.
- They do not show: That cancer is generally contagious, that this kind of transfer is common, or that both twins must develop cancer after sharing cells.
- They do not establish: The precise timing, sample counts, or detailed technical limitations of the new study from the abstract information available.
- They do not recommend: A screening or treatment action for readers; the study reports a biological reconstruction, not clinical guidance.
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