SIZE: 0.1 - 0.2 mm
TIME PERIOD: 5 - 6 days post-ovulation
The pressure of the blastocele expanding in the middle of the blastocyst against the rigid wall of the zona pellucida, induces "hatching" of the blastocyst from the zona around the sixth day after fertilization.
As the blastocyst enters the uterus free from the zona, the outer layer of trophoblast cells secrete an enzyme to erode the epithelial lining of the uterus and allow the blastocyst to implant.
Human chorionic gonadotropin (hCG) is also secreted by the trophoblast cells stimulating the corpus luteum to continue progesterone production. Progesterone maintains the blood rich lining of the uterus. Endometrial glands in the uterus had already begun to enlarge in response to the progesterone stimulated to release by the corpus luteum ("yellow body") which once surrounded the egg as it grew in the ovary almost six days prior.
The uterus is therefore swollen with new blood capillaries and the circulation between mother and blastocyst begins, a process needed for the continuation of pregnancy.