Identical Twins Share Cancer in Womb, Study Finds

A medical report published last week details an exceptionally rare instance of cancer transmission between identical twins during pregnancy. Researchers at the Wellcome Sanger Institute and collaborating institutions found that a tumor originating in one fetus spread to its sibling through their shared placental blood supply. Both infants died shortly after delivery, but the genetic analysis offers new insights into twinning mechanisms.

Cancer Transmission Pathway

Doctors initially identified a facial mass on one twin during fetal imaging scans. This discovery prompted early delivery via caesarean section at 34 weeks of gestation. Post-mortem examinations revealed widespread tumors in both children. With family consent, scientists sequenced the entire genome of cells from the newborns and their tumors to trace the disease's origin.

The study, published in Nature Communications, concluded that the cancer began in Twin A, likely from the facial mass, and subsequently metastasized throughout Twin A’s body. It then crossed over to Twin B via the placenta. The researchers noted that this transmission likely occurred late in gestation and happened only once, indicating it was a rare event rather than an inevitable outcome.

Genetic Analysis Findings

Although identical twins share the same genetic makeup, unique mutations accumulate rapidly in each individual. This divergence allowed the team to reconstruct the specific genetic paths taken by the children and their cancers. The work delineates a method for mapping tumor spread in utero while also reconstructing the earliest steps from zygote to genetic segregation of twins.

Beyond the oncological findings, the genetic data revealed unexpected biological details about the twins themselves. The placenta consisted almost entirely of cells from Twin B. Evidence suggests Twin A originated from a single cell early in the zygote’s development before splitting into two distinct embryos.

Challenges to Classic Twinning Models

This pattern contradicts the classic model of twinning, which predicts that half of the pre-split embryo’s cells would form each twin and that the placenta would contain a roughly even mix of cells from both. Sam Behjati, a researcher at the Wellcome Sanger Institute in Cambridge, told New Scientist on Wednesday that placenta experts would find such a distribution highly unusual.

Behjati stated that the mother insisted on learning from the tragedy, calling her contribution an "amazing gift to science." The incident provided answers regarding the babies' condition and offered fundamental insights into twinning. The research team plans to continue studying twin genetics to determine if this phenomenon is more common than currently assumed.