Estrogen, secreted more heavily during the first 2 weeks of the menstrual cycle, is primarily responsible for triggering increased mitosis of endometrial cells, thus stimulating regenerate in size of the endometrium. This is advantageous because the superficial lining of the endometrium is sloughed at the end of each menstrual cycle, that is, during what is termed menses. After ovulation occurs (at or about 2 weeks), cells left behind in the ovary still produce hormones.