Twin to twin transfusion occurs usually only in monozygotic twins, especially where they are monochorionic.
Anatomically there is an artery to vein anastamosis between the twins; the donor becomes anaemic and may show considerable growth retardation, the recipient becomes polycythaemic, hypertensive, and will be heavier than the other. An extreme of this may be intrauterine death in either twin, although the donor is more likely.
Somewhat paradoxically, provided both twins survive to delivery, it can be the recipient which has the rougher postnatal course; the effect of overload can be considerable.
Note that artery to artery anastomosis or vein to vein anastomosis has a different effect; if there are both then an acardiac fetus may develop; under other circumstances the anastomosis may not be noticed.
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