In twin pregnancies problems are most likely to arise if the twin fetuses share a common placenta and thus share their supply of nutrients.
A common placenta has vascular connections (1) through which blood can pass between the twin fetuses. If the blood flow is unbalanced, one fetus may receive too much blood, and the other too little (“feto-fetal transfusion syndrome” or “twin-to-twin transfusion syndrome”). As a result, both fetuses may die. Where the placental area is unevenly distributed between the two fetuses, one fetus may be markedly smaller than the other (2) and eventually stop growing.