A baby girl in Hong Kong was born "pregnant" with twin fetuses, it has been reported.
The baby's condition, known as fetus-in-fetu, has been incredibly rare, occurring in only about 1 in every 500,000 births and it's not clear exactly why it happens, Discovery News reported.
The World Health Organization considered a tiny fetus found within an infant to be a kind of teratoma, or tumor, rather than a normally developing fetus.
But the doctors who treated the baby girl wrote that rather than a teratoma, the tiny fetuses might instead be the remains of sibling twins that were absorbed during the pregnancy.
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. 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. Then, for some mysterious reason, the two smaller fetuses were absorbed into the body of the remaining child.