Twin growth problems: causes and sonographic analysis.

Understanding the significant increased risks associated with twin pregnancies requires knowledge of the embryogenesis of twins and the unique placental characteristics seen only in twin gestations. Dizygotic "fraternal" twins, 70% of all twins, are at relatively low risk when compared with monozygotic twins, largely due to abnormalities seen in association with monochorionic placentation. The sonographic determination of chorionicity and amnionicity allows better estimation of pregnancy risk--up to 50% mortality in monochorionic-monoamniotic twins. Careful evaluation of intrauterine twin growth assists in the early identification of fetal abnormalities because normal twin growth should parallel that of singleton pregnancies until late in the third trimester. A number of the unique complications affecting growth in twin pregnancies are discussed, including twin transfusion syndrome, the "stuck twin" phenomenon, twin embolization syndrome, and development of acardiac twins.