Twin-to-twin transfusion syndrome, or TTTS, is a disease of the placenta. It can affect identical twin pregnancies or higher order multiple pregnancies in which two or more of the babies are monochorionic, or share a placenta.

In TTTS, blood passes disproportionately from one baby to the other via connecting blood vessels within their shared placenta. This means that one baby, the recipient twin, gets too much blood, while the donor twin doesn't get enough blood, putting both at risk for serious injury or death.

"We know that in pretty much every monochorionic pregnancy there are vascular communications between the twins," says Dr. Francois Luks of the Brown University Fetal Medicine Program. "In about 20 to 25 percent of these pregnancies, some degree of TTTS occurs. In about 10 to 15 percent, it's severe."

Both twins try to adapt. The donor twin's body works to conserve fluid and energy, leading to oligohydramnios, or low amniotic fluid, and intrauterine growth retardation, or IUGR. As the condition worsens, the donor twin's fluid output may become so low that he or she is literally "stuck" to the side of the uterus.

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