British researchers have documented an exceedingly rare medical case in which a malignant tumor originated in one identical twin and crossed the shared placental blood supply to the other in the womb. Both newborns presented with tumors and died shortly after birth.
Medical professionals first identified a clinical anomaly during prenatal imaging when scans revealed a distinct mass on the face of one twin. Faced with the developing complication, physicians scheduled an early delivery via Cesarean section at the thirty-fourth week of pregnancy. Both newborns arrived with tumors distributed across their bodies, and both infants passed away shortly after birth.
With the permission of the family, investigators initiated a genetic investigation.
Genetic Sequencing Traces the Tumor Journey
Although identical twins share the same genetic structure, unique mutations accumulate within each individual over time, which allowed researchers to reconstruct the genetic path taken by the children and their cancers. Whole-genome analysis established that the malignancy initiated in the first twin—designated as twin A—beginning with the facial mass before spreading throughout that child’s body.
From there, malignant cells crossed the shared placental circulation to reach the second sibling, twin B. Investigators concluded that this crossing occurred in a late stage of pregnancy and likely took place through a single transfer event, indicating that the inter-twin metastasis was an extraordinary occurrence rather than an inevitable consequence.
Placental Analysis Challenges Conventional Embryology
Beyond tracing the origin of the cancer, the detailed genetic examination of the monochorionic diamniotic pregnancy yielded an unexpected discovery regarding the structure of the placenta. Comprehensive genetic screening revealed that the placenta was composed almost entirely of cells belonging to twin B.
This composition departs from the traditional understanding of twin development, which holds that the cells of a pre-division embryo split into two halves—forming twin A and twin B—and the placenta forms from an intermingled mixture of cells from both.
Alongside mapping tumor dissemination within the uterus, our work provides a robust framework for reconstructing the initial steps that lead from the fertilized egg to the genetic separation of twins. The
The findings suggest that twin A originated from a single cell very early in the developmental timeline as the fertilized egg transitioned into an embryo before dividing into two separate twins. While this remains an interesting discovery, researchers intend to investigate twin genetics further to determine whether this phenomenon is more common than assumed.
Broader Implications for Prenatal Research and Public Understanding
While cancers that begin in the womb are very rare, cases involving identical twins are rarer still. Such phenomena can result from a shared inherited mutation that causes the same type of cancer to form independently in each twin, or from the spread of one twin’s cancer to the other. In this case, researchers concluded the cancer spread from one twin to the other via the shared blood supply in the placenta.
The study authors emphasize that this documented event does not mean cancer is contagious in everyday life.