Doctors in Greece Thursday removed from the belly of a 9-year-old girl what they believe was her embryonic twin absorbed into her abdomen when they were both in the womb.
Andreas Markou, head of the pediatric department at Larissa General Hospital in Athens, said the 2-inch-long embryo removed from the girl's belly was a fetus with a head, hair and eyes, but no brain or umbilical cord, according to The Associated Press.
It is phenomenon called "fetus in fetu," or baby within a baby, said Jay Grosfeld, a professor of pediatric surgery at Indiana University who has written about the "exceptionally rare" condition.
In the first month of pregnancy while developing in the womb, Grosfeld said, one twin "enters into the other through the umbilical cord where it assumes a parasitic position in regard to the host baby. After birth, it presents as an abdominal mass."
In the early days of development, the parasitic twin "goes through that still developing opening in the abdomen. [The embryo] never fully develops, but it is a partially developed fetus that has certain structures like an axial spine and the beginnings of organs," said Grosfeld. "The parasitic twin is neither a true twin nor a conjoined twin."
"This lump or mass palpates in belly," he said. "It appears in images as a calcified structure. You can see the bone structure and axial configuration of the spine."
Grosfeld said that once the mass is removed, children typically recover quickly with no longstanding damage.
"As far as the problems kids can have, it is not that bad," he said. "The child is usually otherwise perfectly healthy."
The girl, whose name has not been released publicly, is recovering well, according to the AP. ABC News could not reach the girl's doctors in Greece.
"[The doctors] could see on the right side that her belly was swollen, but they couldn't suspect that this tumor would hide an embryo," hospital director Iakovos Brouskelis told the AP.
Fetus in fetu is very rare, Grosfeld said. The Greek physicians said it occurred only once in every 500,000 people.
In 1999, a man from Nagpur, India, was treated for a mass in his belly that had been painful for years and had advanced to the point that he appeared to be nine months pregnant.
Sanju Bhagat, then 36, was rushed to the hospital one evening and doctors removed a fetus -- his twin -- from his abdomen.
"Because of the sheer size of the tumor, it makes it difficult [to operate]. We anticipated a lot of problems," Bhagat's surgeon Ajay Mehta of Tata Memorial Hospital in Mumbai told ABC News at the time.
"To my surprise and horror, I could shake hands with somebody inside," he said. "It was a bit shocking for me."
Fetus in fetu in is one of several conditions that can result in an embryo or embryolike mass developing in one's body, doctors told ABC News.
With scant details available in the Greek case, physicians consulted by ABC News offered alternative theories to the fetus in fetu diagnosis.
"There are several ways for a fetus to appear in the body," said Richard Paulson, an obstetrics professor and chief of reproductive endocrinology and infertility at the University of Southern California. "One is an ectopic pregnancy, in which a fetus develops in the fallopian tubes outside the uterus. There are all kinds of ectopic pregnancies, and while I would not want to imply that this girl was sexually active, it is a possibility."
"Another option is a teratoma, which comes from the Greek for monster and tumor," he added. "These tumors grow in the ovaries and are cysts that have hair or bone, and can sometimes contain brain or eye tissue. They don't actually form fetuses."
Another phenomenon that occurs with twins while in the uterus is called "vanishing twins," doctors told ABC News.
"It is pretty well known that one in 80 pregnancies end in twins. The frequency of twins is much higher at conception, around one in eight," said Robert Marion, an OB/GYN and director of the Children's Evaluation and Rehabilitation Center at the Albert Einstein Medical College.
"The question is: What happens to all those other twins?" Marion said. "Only one in 10 twins make it to term. In all other cases one disappears."
"The vanishing twin dies and is usually absorbed by the mother or the placenta," he added. "In some cases, it is not fully absorbed, but compressed. This flattened, mummified fetus can sometimes block the passage of the viable fetus. Parts of the fetus sometimes appear in the placenta. I've never heard of a fetus absorbing the twin, but it is possible."