TTTS (Twin-to-Twin Transfusion Syndrome) is an imbalance in blood exchange between identical twins.
One twin, known as the "donor," gives away more blood than it receives, which creates a risk of organ failure or malnutrition. Because of the loss of blood volume, the donor twin's kidneys don't need to filter out as much fluids, which leads to a decrease in urination. This in turn affects the development of the donor twin's bladder and leads to lower levels of amniotic fluid. A reduction in blood volume in the donor twin's circulatory system can also cause cardiovascular dysfunction, placing the donor at risk of death.
The other twin, known as the "recipient," receives too much blood and is susceptible to cardiac complications due to overworking of the heart. This increase in blood volume ultimately affects the function of the heart muscle, which can lead to cardiovascular dysfunction, heart failure, or death.
It is estimated that 10-15% of identical twin pregnancies are at risk for TTTS, and if left untreated are at a 90% risk for fetal demise. This is why frequent monitoring is so important with identical twin pregnancies. Fortunately, there are treatment options available, known as Fetoscopic Selective Laser Ablation and Selective Cord Occlusion which help to equalize cord flow between the babies.
For additional information on TTTS, please refer to the Sources below.