The follicle phase starts on day one of our cycle a.k.a the first day of your period and continues up to ovulation. During this time the brain is telling the ovaries to prepare an egg to be released.
Neural activity causes the hypothalamus in the brain to release neurohormones that are delivered to the anterior pituitary gland which in turn stimulates the secretion of follicle-stimulating hormone. FSH promotes the development of the follicles in the ovaries.
What are follicles?
They are tiny fluid-filled sacs that produce oocytes (eggs). Follicles also produce the female hormones estrogen and progesterone. These hormones control and trigger processes within the menstrual cycle.
As the ovarian follicles are maturing at different rates, the largest follicle at this time becomes the "dominant follicle". The mature dominant follicle will release an oocyte (egg), this is called ovulation (we'll talk more about that at another time).
Meanwhile, the dominant follicle also produces estrogen which signals for the uterus around day six of the cycle to start thickening its endometrium (lining) in preparation for a fertilized egg. The thickening of the endometrium is essential for successful implantation leading to a pregnancy.
If this doesn’t happen the endometrium is shed at the start of the next cycle which results in a period.