A baby girl born in Hong Kong had a rare, mysterious medical condition of being "pregnant" with twins when born, doctors have reported.

The baby was born with possible absorbed twins inside her, they said, an extremely rare condition known as fetus-in-fetu.

In the condition, a partially developed fetus, one of twins, becomes incorporated and absorbed into a normally developing fetus inside the womb.

The Hong Kong case apparently involved triplets, as two partially formed fetus-like structures weighing less than an ounce each were found between the baby girl's liver and left kidney.

When the baby girl, a healthy 9 pounds at birth, reached three weeks of age the partially developed fetuses were removed, and the girl recovered and has been discharged from the hospital.

Fetus-in-fetu occurs in just one in 500,000 births globally and fewer than 200 cases have ever been recorded in medical literature, doctors said.

"It was almost impossible to detect during the prenatal check-up, as the embryo inside the baby was too small," said Dr. Yu Kai-man, a specialist in obstetrics and gynecology.

"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," he said.

The baby girl and the fetuses shared identical DNA, evidence that the mother was initially pregnant with identical triplets, doctors said.

Each of the removed, partially developed fetuses had a spine, bones with marrow, intestines, "primitive" brain matter and an umbilical cord, they reported.

Their size suggested the fetuses could have been as much as 10 weeks along in their development when they stopped growing and were absorbed.

The mechanism for such "absorption" is not fully understood, the doctors wrote in a study of the birth reported in the Hong Kong Medical Journal.