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Days 1-13: The Follicular Phase
At the beginning of your menstrual cycle the pituitary gland (located at the base of your brain) produces increasing amounts of two hormones – FSH (follicle stimulating hormone) and LH (luteinizing hormone). FSH works on your ovaries to stimulate approximately 10 to 20 egg-containing sacs known as follicles.
At around day five, increasing amounts of LH are released to help these follicles mature. As these follicles in the ovaries mature they begin to produce a hormone of their own called estrogen. Estrogen works to stimulate more LH production which continues the maturation process of these follicles.
Around day eight of your cycle, one of these developing follicles will emerge as the dominate one and all the others will begin to shrivel and disintegrate. Around the twelfth day, the increasing level of estrogen is also causing the lining of your uterus to thicken as it prepares for the implantation of a fertilized egg.