Wanted pregnancies don't happen, the discomfort of PMS makes the days before menstruation very difficult, or menstruation may occur irregularly or not occur at all. In a healthy female body they balance each other so phases of the menstrual cycle occur regularly. When one is going down, the other is going up. When the balance between estrogen and progesterone is lost, your body may act in unpredictable ways. When everything works normally, the menstrual cycle goes through appropriate phases.
Follicles grow in the ovaries.
The tissue in the womb prepares for a fertilized egg. If eggs are fertilized, then the menstrual cycle is suspended for the nine months of pregnancy. The body automatically produces appropriate amounts of both estrogen and progesterone. These in turn stimulate secretions for nourishing the fertilized egg. If fertilization doesn't occur, the nurturing environment created in the womb rapidly changes.