WASHINGTON (CBS DC) — A baby girl was born pregnant with what doctors believed to be two tumors in her abdomen, but were later found to be the fetuses of her own 8- and 10-week twins.

Researchers reported about the November 2010 case of a newborn girl who carried two fetuses in her abdomen before undergoing successful surgery to remove them at just two weeks of age, the South China Morning Post reports. This week in the Hong Kong Medical Journal, researchers described the highly abnormal condition known as “fetus in fetu.”

The fetuses were estimated to be of 8-10 weeks of gestation, each having four limbs, a spine, rib cage, intestines and anus, but differing in weight. One fetus weighed 14.2 grams and the other 9.3 grams, with both having an umbilical cord connected to a single placenta-like mass. The pregnant baby girl made a speedy recovery from the operation was discharged from the Yau Ma Tei hospital just eight days later.

The extremely rare condition occurs among just 1 in 500,000 births worldwide. Fewer than 200 cases have ever been reported in medical literature, with this case being only the second regionally reported instance.

Dr. Yu Kai-man, a specialist in obstetrics and gynecology, told the South China Morning Post it was the first documented case in the city.

“It was almost impossible to detect during the prenatal check-up, as the embryo inside the baby was too small,” said Yu, a former professor at Chinese University of Hong Kong. “Since it is impossible for the little girl to have conceived the pregnancy on her own, the fertilization of the twin fetuses, of course, belongs to her parents, which has gone to the wrong place.”

The World Health Organization, which has an office based in Washington, D.C., has classified the “fetus in fetu” condition as a type of cancer called mature teratoma, but researchers write that “the theory of demised multiple pregnancy has gained much support recently. More evidence is needed to confirm either theory.”

Last year, a 44-year-old “stone baby” was found inside the abdomen of an 84-year-old Brazilian woman – discovered only after the woman complained to doctors of stomach pain.