A new study has detailed the unusual case of an unborn twin that was discovered in the brain of a 1-year-old girl in China. While this case is extraordinarily rare, it’s not the first time it’s been reported in the medical literature.
Doctors at Fudan University in Shangai explained that the young girl presented with an enlarged head and problems with her motor skills. CT scans of her head showed that her brain was compressed and there was a build-up of fluid caused by a significant growth that could be found within the ventricles.
Details in the case report are scant, but it describes that surgical removal was carried out, revealing a rare case of “intraventricular fetus-in-fetu”. In other words, the fetus of the child's unborn twin was inside their skull. The tiny “fetiform mass” appears to have grown upper limbs and even finger-like projections, but remained extremely undeveloped.
Fetus-in-fetu cases such as this are thought to occur during the very early stages of twin pregnancy when the cluster of dividing cells made by a fertilized egg, called the blastocyst, fail to separate properly. The result is one of the early embryos becoming enveloped by the other. Encased by the other twin's replicating cells, it fails to develop but remains "alive" thanks to the twin's blood supply.
It’s unclear how the 1-year-old girl recovered after the tough operation as the case report is extremely brief.
Nevertheless, there have been other similar cases where the child has recovered well. In one study, published in 1982, scientists at the London Hospital reported a 14-centimeter (5.5-inch) long fetus in a 6-week-old child who presented with enlarging head size. The removed fetus also had noticeable features like developing limbs, a head, and a body. After the surgery, the child was said to have enjoyed an “excellent recovery”.
Fetus-in-fetu cases are outstandingly rare. The most reliable figure suggests they occur in every 500,000 births, although it’s unclear how that number was reached. In most incidences, the unborn fetus is found within the body of the live baby, so it’s exceptionally uncommon to find it affecting the brain.
The recent case study was reported in the open-access journal Neurology.