The tragedy of TTTS is that there are two babies (at least) who begin the pregnancy healthy, without genetic defects, who suffer consequences related to their placenta type.
The historic twin survival rate with chronic TTTS was less than 10% before doctors could make the diagnosis in the womb by ultrasound. With the introduction of ultrasound (in 1980), the survival odds greatly improved because treatment of the TTTS was now made possible while the mother was still pregnant (see Warning Signs for the babies).
The excess amniotic fluid (polyhydramnios) would cause over distention of the uterus, and pregnancy loss occurred when the mother went into premature labor or the baby's bag of water broke.
In some circumstances the recipient twin my pass away (from heart failure due to the excess blood), or in other cases the donor (from the loss of blood or having a placental share too small to receive the necessary nutrients). This sometimes made the situation better for the other twin, but in half of the cases the other twin also passed away or survived with severe birth defects.