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 below).
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.