The follicular phase is the first part of the menstrual cycle, beginning on the first day of menstruation and lasting until ovulation. During this phase, the pituitary gland releases hormones that stimulate the growth of several follicles in the ovaries. Each follicle contains an egg, and typically, one follicle will mature fully while the others will stop growing. As the follicles develop, they produce estrogen, a hormone that helps thicken the endometrium, or uterine lining, in preparation for a potential pregnancy. The increase in estrogen levels eventually triggers a surge in luteinizing hormone (LH), leading to ovulation, which marks the end of the follicular phase.