Monochorionic twins can be discordant in amniotic fluid volume,in size,in the presence of congenital abnor malities and in hemoglobin concentration.Severe amniotic fluid discordance is typical for TTTS, which complicates 8–10% of monochorionic twin gestations, usually between 16 and 26 weeks . TTTS is the most important cause of death and handicap in monochorionic twin
pregnancies.Its sonographic diagnosis is based on strict criteria of amniotic fluid discordance.Most European centers adhere to gestational-age-dependent criteria to define polyhydramnios in the recipient sac:deepest vertical pocket 8 cm prior to 20 weeks and 10 cm after
20 weeks.In contrast, in the United States, the 8-cm cutoff is used more often throughout gestation .Both continents agree on the definition of oligohydramnios in the donor’s sac (2cm deepest vertical pocket)