They extract her twin brother’s fetus from a girl’s brainCredits: Zongze Li et al / Neurology.


In an extremely unusual case, a 1-year-old girl had to undergo brain surgery to remove her twin brother’s fetus, which had been “absorbed” during pregnancy, as part of a developmental process called brain folding. step towards the formation of the structure of the brain and spinal cord. This would explain how the fetus ended up inside the sister’s skull: usually, the “parasitic twin” appears in the abdomen region.

An investigation carried out by scientists from Fudan University, in Shanghai, China, reports the performance of a surgical intervention through which a fetus was removed from the brain of a 1-year-old girl. The girl, who had a delay in the development of motor skills, abnormal increase in head circumference and accumulation of fluid in the brain, harbored a “parasitic twin” in her brain structure: when removed, the malformed fetus had upper extremities and even projections in the form of fingers.

Identical twins

According to the new study, published in the journal Neurology, the mass removed from the girl’s head was a “malformed monochorionic diamniotic twin”. placenta, but they had separate amniotic sacs, which are the thin-walled sacs filled with amniotic fluid that “protect” fetuses as they develop. This class of twins is identical in that they come from the same fertilized egg.

The anomaly occurs when one fetus becomes “enveloped” by the other: the phenomenon is known as “fetus in fetu” or “parasitic twin”. In most cases, the absorbed twin stops developing while the other continues to grow normally. This strange phenomenon occurs in about 1 in 500,000 births, but it is even more unusual for the malformed fetus to appear in your brother’s or sister’s brain, as happened on this occasion in China.

Usually, the “parasitic twin” is seen as a mass in the abdomen of the other fetus, immersed in the tissues lining the abdominal wall. However, in this case, it would have started to develop at a very early stage, when the fertilized egg forms a group of cells called a blastocyst: it is an embryo of 5 or 6 days of development, which presents a complex cellular structure, formed by approximately 200 cells.

An almost unique case

In this case, as can be seen in a post on Twitter by specialist Oren Gottfried, from Duke University, in the United States, who did not participate in the study, the “parasitic twin” fetus showed clear signs of development. Studies revealed that the fetus contained a spine and two leg bones, specifically the femur and tibia, along with “buds” in the form of upper limbs and fingers.

In addition, he suffered from spina bifida, a condition in which part of the spinal cord is exposed rather than covered by tissue from his back. Everything indicates that the fetus lodged in the brain during a developmental process called neural plate folding, an essential step in the formation of the structure of the brain and spinal cord. Only about 200 cases of fetus in fetus have been scientifically documented, of which only 18 occurred inside the skull.

According to an article published in the Miami Herald, the extracted fetus was about four inches long. DNA tests confirmed it was twin of the girl, which continued to grow inside her sister, aided by the regular supply of blood. In 1982, a study reported a similar case in which a 14 cm long fetus, complete with limbs, torso, head and other recognizable features, had to be removed from the skull of a 6 week old child. .


Teaching Neuroimaging: Intraventricular Fetus-in-Fetus with Extensive De-De novo Gain in Genetic Copy Number. Zongze Li et al. Neurology (2022). DOI: