Women are supposed to have two hormones, estrogen and progesterone.
Estrogen stimulates the uterus to grow.
Progesterone stops the stimulation. If a woman has estrogen without progesterone, her uterus is stimulated all the time which can lead to uncontrolled growth which is cancer. If a woman lacks both estrogen and progesterone, she is at risk for breaking her bones. Structural abnormalities of the uterus and vagina can interfere with menstruation, but most of the time, irregular periods are caused by abnormal ovarian function. There are four types of irregular periods.