The world of twins: an update.

In last years, owing to the widespread availability of assisted-reproduction technology, multiple pregnancy rates in Western countries have increased. In twin pregnancies, an increased rate of gestational complications, intrauterine growth restriction (IUGR), preterm birth and severe perinatal conditions is present. These complications are more frequent in monozygotic twins compared to dizygotic twins as well as an increased relative risk of chromosomal abnormalities and congenital malformation. Monochorionic twins are at higher risk for complications, since they share a common placenta where an imbalance in unidirectional arteriovenous anastomoses can lead to twin–twin transfusion syndrome. This extremely dangerous condition, if not early identified, can determine severe fetal complications with mortality rates that, in case of no treatment, reaches 90%. Laser photocoagulation is the treatment of choice in severe twin-to-twin transfusion syndrome with high survival rate. IUGR occurs more frequently in MC twins and along with prematurity and perinatal pathology is considered an important determinant of developmental delay.