CHINA -- A woman was already pregnant, when she got pregnant again.
The woman thought the two babies were twins after she underwent in vitro fertilization, because she was a surrogate for a couple in China.
The babies were born and given to the couple. But a month later, they realized the babies looked too different from each other.
A DNA test revealed one of the babies was naturally conceived -- even though she was pregnant already.
The surrogate was eventually given her baby, who's three weeks younger than the other child.
Doctors say the event is so rare, they don't know much about it.