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The lining of the uterus, called the endometrium, thickens during each menstrual cycle in response to the hormone oestrogen. Progesterone levels begin to rise around the time of ovulation, changing the endometrium to make it ready for the implantation of a fertilised egg. If conception doesn't take place, the levels of progesterone and oestrogen fall, causing the endometrial lining to be shed.