They were both joined to a placenta-like mass by umbilical cords. Each one had four limbs, skin, a ribcage, intestines and primitive brain tissue, according to a study published this month in the Hong Kong Medical Journal
It was "one of those very rare things that make the world stand still," said Dr. Nicholas Chao, one of the surgeons who operated on the baby. He said he'd never seen anything like it before during his career in pediatric medicine.
The unusual condition, known as "fetus-in-fetu," is estimated to happen once in every half-million births but has been reported fewer than 200 times worldwide, according to the study published by Chao and his colleagues.
The baby, weighing about 9 pounds, was born in November 2010 to a woman from mainland China. The little girl recovered well from the operation to remove the mass of tissue, Chao said Tuesday.
But uncertainty remains over how the two unusual entities ended up inside her and whether they can even be classed as fetuses.
"There are controversies over what these things really are," Chao said, explaining that they might be considered as other fetuses that had been in gestation or as a very mature type of tumor, known as a teratoma.
In the case of teratomas, the cells divide and acquire the maturity of the different tissues.
"Both theories are sound," Chao said, and there's "not enough hard scientific data to prove either one because of the limited cases."
Spines, genitalia but no skulls
The case that he and his colleagues encountered appears to add some weight to the fetus argument, though.
Twin fetuses-in-fetu are particularly rare, he said.
The two forms had somewhat different weights but identical levels of organ development, consistent with about 10 weeks of gestation. They had spines and "ambiguous external genitalia" but no skulls.
The mother of the girl had reportedly had a normal ultrasound early in the pregnancy, suggesting that the other two fetuses "might have been tiny parasitic fetuses that had grown slowly" with the girl, according to the study.
That contrasts with the more popular theory of additional fetuses that develop normally early on and then get absorbed by the main fetus and stop growing.
Parents surprised at first
Because most of the mother's care during pregnancy took place in mainland China, the Hong Kong doctors said there were no scans showing what happened during the second trimester.
The mass of tissue was spotted in a scan at 37 weeks. Further scans after the baby girl was born showed the mass between her spleen and left kidney, measuring nearly 2 inches across.
Her parents showed "some initial surprise" about the diagnosis, Chao said. But the condition wasn't seen as life-threatening, and the operation went smoothly.
Doctors, meanwhile, face a long wait before they get a clearer idea of how the mysterious "fetuses" are formed.
"You need a big series of these cases and lots of observations to get a more scientific understanding," Chao said.