A baby girl in Hong Kong was born pregnant with twin fetuses, a condition known as fetus-in-fetu. It is an incredibly rare phenomenon, and occurs in only about 1 in every 500,000 births.

One of the fetus weighed 0.3 ounces (9.3 grams), while the other 0.5 ounces (14.2 grams), corresponding to about 8 and 10 weeks' gestation respectively. Both the babies had an umbilical cord that linked to a placenta-like mass in the little girl's belly.
The baby girl was too young to have conceived the fetuses herself. Instead, it is likely that the girl was once one of triplets, and for some mysterious reason, the two smaller fetuses were absorbed into the body of the remaining child.
Source-Medindia