Fetus Developed Limbs, Bones, Fingernails While Growing Inside 1-Year-Old Twin Sister’s Skull in China

A 1-year-old sister had a twin that grew in her skull. It's a rare medical anomaly that has only been recorded a handful of times but happened to China at least twice.

Fetus Grows Inside Its Sister's Skull

Parents from China took their daughter to the hospital due to her enlarged head and problems with her motor skills. CT scans showed a four-inch-long fetus inside her skull.

The fetus had already developed upper limbs, bones, and even fingernails, suggesting that it had been growing for months inside its sibling while in the womb, Daily Mail reported.

The CT scans also showed that the sibling was pressed against her brain, and the child had hydrocephalus due to the build-up of fluid in the brain, which caused an enlarged head, extreme sleepiness, and seizures.

According to the doctors, the fetus continued survival because it shared a blood supply with its twins.

It was unclear if the surviving twin would suffer long-term damage.

According to Dr. Zongze Li, a neurologist at Huashan Hospital, Fudan University, who treated the girl, the intracranial fetus-in-fetu is proposed to arise from unseparated blastocysts. The conjoined parts develop into the forebrain of the host fetus and envelop the embryo during neural plate folding.

What Is Fetus-in-Fetu?

According to doctors, it was a case of fetus-in-fetu caused by incomplete separation when identical twins, who form in one egg, split.

There are theories that the healthy twin connects with the mom via the placenta while the other latches on the sibling's blood vessels.

As the bigger twin grows, the smaller one becomes absorbed into their abdomen. However, others suggested that it happens due to late cell division.

The unviable fetus may continue to develop inside its twins for several weeks or months, even forming organs, bones, and limbs.

The newest story was revealed in December in the American Academy of Neurology's journal Neurology.

It was one of the only 18 reported cases in medical literature but not the first in China. Another case was reported in 2015 where the unborn fetus was found inside the scrotal sac of its male twin.

The infant was only 20 days old when the parents took him back to the hospital because his scrotum started to swell. The scans showed a well-defined mass complete with bones and buds, which the doctors said would have formed into limbs.

The fetus was removed via surgery, and its twin was discharged five days after surgery after fully recovering.

Doctors in Thailand also discovered a similar incident in 2017. They found three siblings inside the skull of an unborn girl, and each had multiple well-developed organs, including nervous, digestive, and respiratory systems. The doctors said they were connected to the host fetus via a single artery and vein, the umbilical cord.

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