At this stage, the embryo appears as a bilaminar disc composed of epiblasts and hypoblasts. In certain cases, the embryo may not implant in its normal position or may be unable to implant. This may be caused by mechanical block in the Fallopian tube such as a diverticulum, constriction due to infection, or previous surgery and adhesions, tumors. It may also be related to abortion and the inability of the embryo to establish a dialogue with the endometrium and to implant. Such cases lead to spontaneous abortion which occurs at about 45% of pregnancies.