In an extremely rare case, doctors surgically removed a fetus from the brain of a 1-year-old child. Doctors found the fetus after the child showed delayed development of motor skills, an increased head circumference and a buildup of fluid in the brain.
The report was published in the journal on December 12, 2022 neurology (opens in new tab), noted that the mass in the young child’s head was a “malformed monochorionic diamniotic twin,” meaning that the fetuses in the womb had once shared the same placenta but had separate amniotic sacs, the thin-walled, fluid-filled sacs that make up the fetuses surround as they develop. These types of twins come from the same fertilized egg, meaning they are identical.
The anomaly in which one fetus is enveloped by another is known as a “fetus within a fetus” or sometimes “parasitic twin”. The absorbed twin usually stops developing while the other continues to grow, the The Miami Herald reports (opens in new tab).
The phenomenon occurs in an estimated 1 in 500,000 live births (opens in new tab); usually the malformed fetus appears as a Mass in the abdomen of the other fetus, pinched behind the tissues lining the abdominal wall. In this case, however, the mass appeared in the head of the “host” fetus and probably arose very, very early in development, at the stage when the fertilized egg forms a clump of cells called a blastocyst.
Related: Have you shared the womb with a ‘disappearing twin’? The answer may be written in your DNA.
“It is suggested that the intracranial fetus arises in the fetus from non-separated blastocysts,” meaning that the clumps of cells destined to grow into two separate fetuses remained attached to each other, the case report authors wrote. “The connected parts develop into the forebrain of the host fetus and envelop the other embryo during neural plate folding.” (The neural plate is a structure that forms early in development and gives rise to the nervous system.)
“Intracranial fetus within fetus is extremely rare with <20 reports published worldwide," a 2020 case report in the journal World Neurosurgery (opens in new tab) written down.
Brain scans of the 1-year-old’s head showed that the fetus contained a spine and two leg bones (femur and tibia) and that the malformed fetus had spina bifida, a condition in which part of the spinal cord is exposed, rather than tissue of the back being covered due to an issue during development. After removal, it was also noted that the fetal mass had “upper limbs and finger-like buds.”
The brief case report does not provide details of the 1-year-old’s condition after surgery.