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 and it is not clear exactly why it happens.
The World Health Organization (WHO) 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, "Rather than a teratoma, the tiny fetuses might instead be the remains of sibling twins that were absorbed during the pregnancy."
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.