Fetus in fetu is a rare condition in which a fetus becomes enveloped by its twin in the womb. While fraternal twins result from two separately fertilized eggs, identical twins are the product of a single fertilized egg that separates. The separation of that fertilized egg doesn't always go smoothly; conjoined twins are one example of an imperfect separation. Fetus in fetu, which occurs very early within a pregnancy, results when instead of separating, one fetus becomes trapped inside the other one. The trapped fetus begins acting like a parasite on its host, tapping into the twin's blood supply to stay alive. If the fetus grows, it can cause great pain for its carrier.
Most cases of fetus in fetu are caught when the host twin is an infant, though there have been cases of people in their 30s and 40s carrying their own twin. One notable case occurred in India: A 36-year-old man looked visibly pregnant, but when doctors operated on what they thought was a tumor, they found a malformed fetus with limbs, genitalia and hair [source: ABC News]. This man's example is a rare scenario, though, as about 90 percent of cases are found before an infant is 18 months old [source: Hoeffel et al.]. The babies usually have an abdominal mass, though sometimes the lump can be found other places, such as the brain or the scrotum.
For those of you eyeing your full-length mirror, bear in mind that this condition is extremely rare, with fewer than 100 examples documented in medical literature [source: Newsome]. Doctors estimate that it happens at 1 in every 500,000 births [source: Goldman]. The scenario does happen in equal rates to males and females, and though it usually involves only one fetus, sometimes multiple fetuses are absorbed. When they are removed, the fetuses are about 1.5 to 9.5 inches (4 to 24.5 centimeters) in length and weigh between 0.04 ounces (1.2 grams) and 4 pounds (1.8 kilograms) [source: Hoeffel et al.]. While various body parts may be present, the fetus is always anencephalic, meaning that it's missing major parts of its brain and skull. Once the mass is removed, the host twins are usually completely fine.
It's worth noting that some researchers believe that what may appear to be a case of fetus in fetu might actually be a teratoma. A teratoma is kind of tumor that can also look like a malformed fetus, yet it's just a group of cells that can develop skin and teeth. Sometimes there may not be enough evidence to tell which has occurred, though an instance of teratoma is more common than that of fetus in fetu. Currently, if the fetus has some evidence of a vertebral column, the case is considered to be fetus in fetu.