Twins 4D ultrasound performance, early pregnancy ultrasound scanning can clearly show two chorionic sac, early pregnancy late in the two fetal implantation site is farther away from the two separate placentas can be seen, two fetal implantation is closer to the two placentas fusion occurs, but the two placentas between the two twin peaks, can be seen in two fetuses. In another type, only one chorionic sac and one placenta are seen in the uterus, with no twin peaks, and two embryos or fetuses are seen in the sac. There is also the monochorionic sac, monoamniotic sac twin fetus, which is the result of the embryo duplicating into two fetuses after the formation of the amniotic sac.