After ovulation, the follicle becomes the corpus luteum. The cells of the corpus luteum produce oestrogen and progesterone. It is progesterone which is responsible for the maintenance of the uterine lining. If you do not become pregnant, the corpus luteum regresses about two weeks after ovulation. Because of this, oestrogen and progesterone levels drop and the stimulation for the endometrium is lost. This causes the shedding of the lining and a new menstrual cycle to begin.
If you become pregnant during a menstrual cycle, fertilization will occur within 24 hours after ovulation. Five days after fertilisation, the embryo (fertilized egg) enters your uterus and becomes embedded in the endometrium a few days later. With implantation, cells that will eventually become the placenta begin to produce human chorionic gonadotropin (HCG), which is sometimes called the "pregnancy hormone." HCG interrupts your menstrual cycle by providing continual stimulation of the corpus luteum to produce oestrogen and progesterone. This prevents the loss of your endometrium.
Phase Four
During this phase, if you become pregnant, the egg moves into your uterus and attaches to the endometrium. If you are not pregnant, the lining of the uterus is shed through the vaginal opening. Then, a new menstrual cycle begins.
