http://www.livescience.com/49766-baby-pregnant-twins.html
Bit more at the link.
A baby born in Hong Kong was pregnant with her own siblings at the time of her birth, according to a new report of the infant's case.
The baby's condition, known as fetus-in-fetu, is incredibly rare, occurring in only about 1 in every 500,000 births. It's not clear exactly why it happens.
"Weird things happen early, early in the pregnancy that we just don't understand," said Dr. Draion Burch, an obstetrician and gynecologist in Pittsburgh, who goes by Dr. Drai. "This is one of those medical mysteries."
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, the case report said.
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, the researchers said. Then, for some mysterious reason, the two smaller fetuses were absorbed into the body of the remaining child.
The fetuses would likely have still been alive and growing when they were absorbed into the surviving baby's body. Once there, however, their development couldn't proceed normally, Burch said.
"They need placental flow and all that other stuff to really grow," Burch told Live Science.
Fetus-in-fetu may, in fact, be similar to a surprisingly common phenomenon: vanishing twin syndrome, Burch said. In many twin pregnancies, one of the twins is completely absorbed and "vanishes" into the body of the other.
Bit more at the link.