We adopted the definition of implantation site from previous publication (Abdallah et al., 2012). The implantation site is visualized in the sagittal plane as a hyperechoic ring that occupies one side of the implanted gestational sac and protrudes into the endometrial lumen It represents the maternal decidual reaction and the beginning of maternal-fetal circulation. This area is also believed to be responsible for future placental formation and development (Brosens and Gellersen, 2010) Distance between the implantation site and the internal cervical ostium (os). This was taken from the lower end of the hyperechoic trophoblast ring of the gestational sac to the internal cervical os in the sagittal plane.