I don’t often see stories that make me think “cool science” and also “this might be a felony in Texas,” but when I do, it might be time for…
Today’s Moment of Science… Parasitic twin on the brain.
There are a few different ways we can produce a small litter of humans. Fraternal twins are each scrambled from unique genetic omelets, sharing neither a placenta nor an amniotic sac. Identical twins are formed when one fertilized egg splits into two embryos, but there are a few provisos. Cleavage of the embryo the first three days results in fetuses with their own placentas and amniotic sacs. If it’s the last half of that first week, they’re likely to share a placenta. If they split the second week they’re liable to share a placenta and amniotic sac.
Whether it’s toys, bedrooms, or amniotic fluid, the chances for problems go up when kids are forced to share. Risk of cord entanglement increases. Feto-fetal transfusion syndrome is another possible complication; one twin gets a richer portion of nutrients from their shared placenta. This doesn’t work out particularly well for either twin, malnourishing one and overworking the cardiovascular system of the other.
Then there’s what happens when an embryo split is late, and subsequently, incomplete.
The delayed cleavage of an embryo can result in conjoined twins. It had been hypothesized that this results from two fully separated fetuses fusing together. Now it’s more widely accepted to be caused by incomplete fission, in which identical twins never fully separate. Survival rates vary greatly depending on complications, but including stillbirths, most conjoined twins don’t survive the first 24 hours outside of the womb.
Then there are those cases that are interesting even for conjoined twins.
The cause of parasitic twins is suspected to be related to conjoined twins. With conjoined twins, two more or less equally developed fetuses are attached with two minds steering the ship. However, if it looks like one person has a superfluous shoulder and a tit hanging out of their lower back, they may have a parasitic twin.
Then there are those cases that are interesting even for parasitic twins.
Fetus-in-fetu is generally categorized as a type of parasitic twin. But instead of a small chunk of twin hanging onto their side for dear life, the surviving twin is not really but kinda sorta born pregnant. Somewhere early in development, one twin envelops the other twin entirely. They’re not ‘pregnant,’ but they’ve got a fetus hanging out somewhere in them. There’s also speculation that this could be a teratoma, a type of tumor that can grow hair, teeth, and bone. Yum.
The recently published report in Neurology documenting a case of fetus-in-fetu in a one year old girl in China shows pictures of what seems like more than just a tumor. The girl’s parents brought her to be examined with observations of “motor delay and enlarged head circumference.” Scans revealed her compressed brain along with the tadpole human she was incubating, and it was surgically removed.
It has a face, bones, fingernails, and arms that look like it’s striking a pose. Some tumor.
The incidence of fetus-in-fetu is suspected to be less than one in a half a million live births, with less than 200 cases reported in medical literature.
There have been no updates on the medical status of the one year old Chinese girl.
This has been your Moment of Science, shocked that anything in human development ever goes right.
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