The purpose of this study was to evaluate the relationship between the yolk sacs separated or not separated by septum and chorionicity twin pregnancies scanned early in the first trimester. Moreover, to determine the relation between size and morphologic features of the yolk sac and outcome twin pregnancy. Twenty four of 38 spontaneous twin pregnancies were dichorionic, eight--monochorionic diamniotic, five monochorionic twin gestations with discordant sizes and views of yolk sacs, and one a twin ectopic pregnancy (underwent salpingectomy). In all 38 sets of twin two yolk sacs were identified. During the first trimester of dichorionic twin pregnancy, the yolk sacs were always separated by a septum and not separated ("Eight" sign) in monochorionic twin pregnancy. In five cases, one of yolk sacs was abnormally large (> 8 mm) and had thinner wall. Three (all MC) of the five mothers spontaneously aborted during the next 2-3 weeks. In one case of monochorionic twin ectopic pregnancy two yolk sacs were seen normally. The sonographic identification of yolk sacs allows a reliable and simple determination chorionicity twin pregnancy. Moreover, visualization of large size (> 8 mm) of yolk sac can be used as a predictor of abnormal outcome pregnancy.