"Another option is a teratoma, which comes from the Greek for monster and tumor," he added. "These tumors grow in the ovaries and are cysts that have hair or bone, and can sometimes contain brain or eye tissue. They don't actually form fetuses."
Another phenomenon that occurs with twins while in the uterus is called "vanishing twins," doctors told ABC News.
"It is pretty well known that one in 80 pregnancies end in twins. The frequency of twins is much higher at conception, around one in eight," said Robert Marion, an OB/GYN and director of the Children's Evaluation and Rehabilitation Center at the Albert Einstein Medical College.
"The question is: What happens to all those other twins?" Marion said. "Only one in 10 twins make it to term. In all other cases one disappears."
"The vanishing twin dies and is usually absorbed by the mother or the placenta," he added. "In some cases, it is not fully absorbed, but compressed. This flattened, mummified fetus can sometimes block the passage of the viable fetus. Parts of the fetus sometimes appear in the placenta. I've never heard of a fetus absorbing the twin, but it is possible."