Examination of the twin placenta.

Twin birth rates have increased dramatically over the past three decades, and twins currently account for 3% of all pregnancies. Twin pregnancies of any type are at risk for prematurity. In addition, monochorionic twin pregnancies (25-30% of all twin pregnancies) are predisposed to a specific set of complications, including twin-to-twin transfusion syndrome (TTTS), twin reversed arterial perfusion syndrome (TRAP), malformations, and intertwin growth discordance. This article reviews the basic mechanisms underlying the twinning process, the relationship between zygosity and chorionicity, and the various types of twinning. We describe the major complications of monochorionic twinning in association with their reported placental characteristics (or lack thereof). Finally, a rational, evidence-based approach to examination of the twin placenta is presented. It is essential for the pathologist to understand the value, strengths, and limitations of examination of the twin placenta in order to provide a meaningful clinicopathological correlation in complicated (monochorionic) twin pregnancies.