We report on a woman who gave birth to dichorionic diamniotic twins with a birthweight discordancy of 30%. Her placenta exhibited characteristic features. The placental villous tissues were completely separated, but there was marked vascular communication between the two placentas. The lighter twin received less blood than the heavier twin via these vascular communications. Although the details are unknown, this abnormal placental structure may have caused the weight discordancy in this case. Clinicians must pay attention to placental vascular communication as one possible cause of twin discordancy in not only monochorionic but also dichorionic twins.