An rare thing happened at a Hong Kong hospital the other day: Doctors investigating an unidentified mass in an infant girl found that the baby was actually "pregnant" with her sibling twins.
But how does that happen?
As LiveScience reports, it's likely that the two smaller fetuses were absorbed into the body of the remaining child. The condition is called fetus-in-fetu and occurs about once in every 500,000 births.
In the Hong Kong case, exploratory surgery when the baby was about 3 weeks old revealed the underdeveloped fetuses between her liver and kidney. It's believed they had all started out as triplets.
One of the fetuses weighed 0.3 ounces and the other 0.5 ounces, which corresponds to about 8 and 10 weeks' gestation, according to LiveScience. The article mentions that both fetuses had an umbilical cord that linked to a placenta-like mass in the baby girl's belly.