Diamnionic Monochorionic Twin Gestations: An Overview

Monochorionic (MC) twins account for about 20-30% of all twins, but contribute disproportionately to mortality, intrauterine growth restriction, and preterm delivery compared with dichorionic (DC) twins. This higher mortality in MC twins is likely due to the effects of placental morphologic characteristics, which include complex vascular communications between the twins associated with twin-twin transfusion syndrome (TTTS), and the tendency for the common placenta to be shared either symmetrically or asymmetrically. In assessment of clinical outcomes for TTTS, artery to vein anastomoses in the absence of artery to artery or vein to vein, especially if present with placental asymmetry, carry the worse prognosis. Chorion status in twins forms the basis for clinical risk assessment and can be determined by 7 menstrual weeks using transvaginal sonography. The variable results reported in the literature for intertwin umbilical artery Doppler findings in MC twins may be explained by differences between sonographic and clinical criteria (including differential hemoglobin concentrations) reported by various investigators. Antenatal fetal Doppler assessment of the umbilical artery and cerebral arteries can help distinguish between TTTS and placental insufficiency in MC twins. Significant restriction of fetal growth occurs in about 25% of multiple gestations, accounting for about 17% of all growth-retarded infants. Redistribution of fetal blood (brain-sparing effect), as determined by Doppler interrogation of fetal cerebral and umbilical arteries, occurs more commonly in MC twins compared to DC twins and in growth-restricted MC twins compared to nongrowth-restricted MC twins. Overall, the prognosis is poorer for the donor twins in TTTS and there is a greater prenatal death rate for the donor (18-35%), and a higher overall survival rate for recipients following fetoscopic laser treatment. Finally, the clinical and sonographic findings suggest that the polyhydramnios/oligohydramnios sequence seen in MC twins likely represents a spectrum strongly linked to placental variables.