A monochorionic twin pregnancy with selective intrauterine growth restriction (IUGR) of one twin is defined as one twin with an estimated fetal weight below the 10 th percentile for gestational age. Selective IUGR occurs in about 12% of twin pregnancies. The incidence of IUGR is similar in dichorionic and monochorionic twin pregnancies, but the risk of neurologic damage to the fetus is greater in monochorionic twin pregnancy. Monochorionic twins have the highest risk of complications, because

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... e well-being of one fetus crucially depends on that of the other because of vascular anastomoses in the common placenta. The spontaneous demise of the twin with selective IUGR may result in the concomitant demise of the twin who is appropriate for gestational age (AGA) in up to 40% of cases or in neurologic damage of the AGA twin in up to 30% of cases. However, even in the absence of single intrauterine fetal death, the risk of neurologic damage is still increased in monochorionic as compared with dichorionic twin pregnancies. Because monochorionic twins with selective IUGR lack a definite treatment process as in twin-twin transfusion syndrome, this article reviews the ultrasound diagnosis, etiologies, Doppler findings, classification, and the management of such high risk pregnancies. KEY WORDS -intrauterine growth restriction, monochorionic twin, umbilical artery Doppler, placental share ■ J Med Ultrasound 2008;16(3):194-201 ■