A medical anomaly that has been recorded only a handful of times led doctors to surgically remove a fetal twin from the skull of its one‑year‑old sister. The fetus, which had developed upper limbs, bones and fingernails, measured about four inches in length. It was discovered when the parents took their daughter for hospital scans because she had an enlarged head and motor‑skill problems.
The condition, known as foetus‑in‑foetu, occurs when twins fuse together in the womb. Only about 200 cases have been documented worldwide, and just 18 of those involved the skull. Researchers believe it results from incomplete separation when a single egg splits or from late cell division. In this case, the surviving twin lived for a year after birth thanks to a shared blood supply, but it remains uncertain whether she will suffer long‑term damage.
Dr. Zongze Li of Huashan Hospital, Fudan University, who treated the girl, explained that “the intracranial foetus‑in‑foetu is proposed to arise from unseparated blastocysts.” Similar cases appear in medical literature. In 2017, Thai doctors found three fetal siblings inside the skull of an unborn girl, connected by a single artery and vein. In 2015, Chinese physicians discovered a fetal twin inside the scrotal sac of a male twin. All of these fetuses were successfully removed by surgery, and the surviving twins made full recoveries.
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