Updated by: Arabinda Naik
Occasionally the blastocyst implants close to internal OS the function of the zona pellucida is to prevent the implantation of the blastocyst at an abnormal site. It does not expose the sticky and phagocytic cells of the trophoblast till the blastocyst reaches the proper implantation site. As the blastocyst forms, the trophoblast excretes enzymes that begin to degrade the zona pellucida and it becomes thinner and finally disappears. However, in one to two percent of cases, the embryo implants either outside the uterus (an ectopic pregnancy) or in a region of uterus can create complications for the pregnancy. If the embryo implants in the inferior portion of the uterus, the placenta can potentially grow over the opening of the cervix, a condition call placenta previa .