Untreated TTTS has serious consequences for each twin and for the whole pregnancy. TTTS complications are caused by their unbalanced connections in the placenta, from prematurity because of preterm labor, or a combination of these factors. If TTTS is detected in the first 20 weeks of pregnancy and is left untreated, there is a very high chance that neither twin will survive.
In TTTS, each twin tries to adapt. The donor twin tries to save water and energy. As a result of these two effects, the donor has low urine output resulting in low amniotic fluid volume (oligohydramnios) and poor fetal nutrition, resulting in intrauterine growth restriction (IUGR). The recipient tries to get rid of excess fluid by increased urination- this leads to excess amniotic fluid (polyhydramnios). The recipient also has to deal with the excess blood cells, which put severe stress on the fetal heart and lead to many complications of increased blood volume and abnormal blood thickness (hyperviscosity).
As this imbalance worsens, the donor is at risk for abnormalities due to compression (being squashed due to low amniotic fluid), from effects of failing placental function (low oxygen can cause brain damage, circulatory collapse and many other permanent effects) and the long-term effects of malnutrition. These effects may be made even worse if the overstretching of the uterus (by the excessive amniotic fluid produced by the recipient) worsens placental function or if preterm labor starts. In severe TTTS, the donor twin is very fragile and cannot cope with the added stress of prematurity.
The risks of the recipient are mainly due to volume overload. The first circulatory effect may be thickening of the heart muscle, which can progress to heart failure and complications affecting all of the baby's systems. These effects on the heart can last into newborn life and may be made much worse if birth is premature.
Both twins. These circumstances are suboptimal for normal development of either twin and may account for the increased rate of developmental delay observed in monochorionic twins at 2 years of age. The placental anastomoses that are responsible for this degree of cardiovascular imbalance carry an additional danger in the event that one twin dies. When this happens, the surviving twin can lose a large amount of blood volume across the connecting vessels into the dead twin. This may cause a sudden drop in blood pressure in the surviving twin, which can result in a heart attack or a stroke. Thus the fate of one twin remains linked to the other through the placental anastomoses.
Left untreated, TTTS will often worsen. The recipient tries to deal with excess fluid by creating more and more amniotic fluid - this will overstretch the uterus. This may cause the mother discomfort and may put pressure on the cervix, the lower part of the uterus. With continuing pressure the cervix may open or the membranes may rupture resulting in miscarriage or preterm delivery. This often occurs at an early gestational age where the chances for survival are poor. Even if the babies might survive the dangers of prematurity, their complications from TTTS may lead to permanent injury or even death.
Maternal risks. Of course, loss of a wanted pregnancy is very serious for the family. There are physical risks to the mother from the overdistended uterus, from the attempted treatments, and even from the delivery, that all must be considered when making decisions about management of TTTS.