Luteal phase - premienstrual phase, corpus luteum, HGC, endometrium

After ovulation, the follicle becomes the corpus luteum. The cells of the corpus luteum produce estrogen 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 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 fertilization, the empyo (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 produceprogesterone. This prevents the loss of your endometrium.

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.