A baby born in Hong Kong was pregnant with her own siblings at the time of her birth, according to a new report of the infant's case. 

The baby's condition, known as fetus-in-fetu, is incredibly rare, occurring in only about 1 in every 500,000 births.  It's not clear exactly why it happens. 

"Weird things happen early, early in the pregnancy that we just don't understand," said Dr. Draion Burch, an obstetrician and gynecologist in Pittsburgh, who goes by Dr. Drai. " This is one of those medical mysteries." 

The World Health Organization considers a tiny fetus found within an infant to be a kind of teratoma, or tumor, rather than a normally developing fetus. [ Seeing Double: 8 Fascinating Facts About Twins]

But the doctors who treated the baby girl wrote that rather than a teratoma, the tiny fetuses may instead be the remains of sibling twins that were absorbed during the pregnancy. 

Missed in check-up

The newborn baby was referred to Dr. Yu Kai-man, an obstetrician and gynecologist at Queen Elizabeth Hospital in Hong Kong, because the baby was suspected to have a tumor, according to the case report.  The mother's prenatal ultrasound had revealed an unusual mass within the infant, but it was unclear to the doctors exactly what the mass was.  During surgery, which was done when the girl was about 3 weeks old, the surgeons discovered two fetuses between her liver and her kidney. 

One fetus weighed 0.3 ounces (9.3 grams) and the other 0.5 ounces (14.2 grams) — corresponding to about 8 and 10 weeks' gestation, the case report said. 

Each of the babies had an umbilical cord that linked to a placenta-like mass in the girl's belly. 

The baby girl was obviously too young to have conceived the fetuses herself.  Instead, it's likely that the girl was once one of triplets, the researchers said.  Then, for some mysterious reason, the two smaller fetuses were absorbed into the body of the remaining child. 

The fetuses would likely have still been alive and growing when they were absorbed into the surviving baby's body.  Once there, however, their development couldn't proceed normally, Burch said. 

"They need placental flow and all that other stuff to really grow," Burch told Live Science. 

Vanishing twins? 

Fetus-in-fetu may, in fact, be similar to a surprisingly common phenomenon: vanishing twin syndrome, Burch said.  In many twin pregnancies, one of the twins is completely absorbed and "vanishes" into the body of the other. 

"When you do a delivery and you see an extra placenta and a cord, you say, 'Oh, it must have been a twin,'" Burch said. 

Fetus-in-fetu has been reported in about 200 cases in the medical literature.  In 2006, doctors in Pakistan removed two fetuses from a 2-month-old girl named Nazia, according to NBC News.  And in 2011, an 18-year-old boy had his retained twin removed in a major surgery, the Seattle Post-Intelligencer reported. 

In rare instances, fetuses that die in utero can become calcified and turn into stone.  In August 2014, doctors in India removed a lithopedion, or stone baby, that a 60-year-old woman had carried in her body for 36 years.  She went to the doctor complaining of abdominal pain and a lump in her lower belly. 

Follow Tia Ghose on Twitter and Google+.  Follow LiveScience @livescience, Facebook & Google+.  Originally published on Live Science.