The menstrual cycle is practically controlled by a system of hormones that is necessary for reproduction, and when the hormone reaches a heightened level, something called estradiol is made, then the stimulation of the ovaries by a luteinizing hormone.
Once that hormone begins developing, the ovaries make an egg that quickly becomes an ovum. The ovary then releases one egg or two during ovulation. The endometrium (the part that sheds its own cells for the menstruation) peaks after ovulation and changes the lining of the uterus to prepare for the hectic process of pregnancy and child labor.
Hope that was helpful.