 Introduction/Background The incidence of twin pregnancies has been rising, largely attributable to the increasing use of artificial reproductive techniques [1]. Compared with singletons, twin pregnancies carry a higher risk of complications [2]. These include maternal hypertensive disorders, growth disturbances, preterm labor, premature rupture of membranes, and other conditions, some of which are unique to monochorionic (MC) twins such as twin-twin transfusion syndrome (TTTS) [3-5]. Compared with singletons, twins face an approximately 5-fold increase in fetal death and a 7-fold increase in neonatal death, primarily due to complications of prematurity [2]. Multiple gestations are also at a higher risk for congenital anomalies, placenta previa, and vasa previa. Importantly, chorionicity determines the prognosis of multiple gestations [2]. People carrying twin or higher-order multiple pregnancies will typically undergo more ultrasound (US) examinations than those carrying singletons [9]. Most will, at minimum, undergo a first trimester dating scan, a nuchal translucency (NT) scan at 11 to 14 weeks, an anatomy scan at 18 to 22 weeks, and 1 or more scans in the third trimester to assess growth. MC pregnancies undergo more frequent follow-up, influenced by the presence of complications and institutional practice [2,9,13]. Some of these high-risk pregnancies will be candidates for fetal intervention. Special Imaging Considerations First trimester fetal anatomy evaluation and fetal echocardiography are available at many institutions [14-16]. Advancement of US technology has allowed improved imaging at earlier gestational ages making this feasible. Indications for detailed first trimester obstetric US were summarized by American Institute of Ultrasound in Medicine (AIUM) and endorsed by other key societies, with relevant International Classification of Diseases, 10th Revision (ICD-10) codes provided [15,16]. Reprint requests to: publications@acr.org
Multiple Gestations.  Introduction/Background The incidence of twin pregnancies has been rising, largely attributable to the increasing use of artificial reproductive techniques [1]. Compared with singletons, twin pregnancies carry a higher risk of complications [2]. These include maternal hypertensive disorders, growth disturbances, preterm labor, premature rupture of membranes, and other conditions, some of which are unique to monochorionic (MC) twins such as twin-twin transfusion syndrome (TTTS) [3-5]. Compared with singletons, twins face an approximately 5-fold increase in fetal death and a 7-fold increase in neonatal death, primarily due to complications of prematurity [2]. Multiple gestations are also at a higher risk for congenital anomalies, placenta previa, and vasa previa. Importantly, chorionicity determines the prognosis of multiple gestations [2]. People carrying twin or higher-order multiple pregnancies will typically undergo more ultrasound (US) examinations than those carrying singletons [9]. Most will, at minimum, undergo a first trimester dating scan, a nuchal translucency (NT) scan at 11 to 14 weeks, an anatomy scan at 18 to 22 weeks, and 1 or more scans in the third trimester to assess growth. MC pregnancies undergo more frequent follow-up, influenced by the presence of complications and institutional practice [2,9,13]. Some of these high-risk pregnancies will be candidates for fetal intervention. Special Imaging Considerations First trimester fetal anatomy evaluation and fetal echocardiography are available at many institutions [14-16]. Advancement of US technology has allowed improved imaging at earlier gestational ages making this feasible. Indications for detailed first trimester obstetric US were summarized by American Institute of Ultrasound in Medicine (AIUM) and endorsed by other key societies, with relevant International Classification of Diseases, 10th Revision (ICD-10) codes provided [15,16]. Reprint requests to: publications@acr.org