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A facial mass detected before birth prompted the early delivery of identical twin sisters. A fetal MRI at 33 weeks showed the mass on one baby's face. When the sisters were born a week later, tumors were found in both. Two children had the same cancer, raising a question that their shared beginnings alone could not answer. Had the disease started separately in each sister, or had cancer cells crossed from one to the other before birth? Researchers turned to a record hidden within their cells. DNA changes, accumulated as their bodies developed, allowed the team to reconstruct events that had happened months before anyone saw a tumor. The study in Nature Communications shows that the cancer began in one sister and passed to the other in the womb, most likely through a single transfer late in the second trimester. The same investigation uncovered clues to how the sisters themselves had formed. The two girls, Amelia and Amaya, died soon after birth. Their family consented to post-mortem examinations for research. The authors dedicate the study to the sisters, whose mother wanted them acknowledged by name.