Monochorionic diamniotic twin pregnancies: natural history and risk stratification.

About 30% of monochorionic twin pregnancies are complicated by twin-to-twin transfusion syndrome (TTTS), isolated discordant growth, twin anemia-polycythemia sequence, congenital defects or intrauterine demise. About 15% will be eligible for invasive fetal therapy, either fetoscopic laser treatment for TTTS or fetoscopic or ultrasound-guided umbilical cord coagulation for a severe congenital defect in one twin or severe discordant growth with imminent demise of the growth-restricted twin. Ultrasound examination in the first and early second trimester can differentiate the monochorionic twins at high risk for adverse outcome from those likely to be uneventful, which may be useful for patient counselling and planning of care.