Hormonal Regulation in Females
The female reproductive cycle involves two sets of physiological changes occurring simultaneously over a 28 day period:

a) The ovarian cycle that culminates in release of mature ovum
b) The uterine cycle which causes the old endometrium to be shed (menstruation) and formation of fresh endometrium in preparation for receiving the embryo.

While the ovarian cycle is driven by two sets of gonadotropins: the Follicular stimulating Hormone (FSH) and Leutenising Hormone (LH), the uterine cycle is regulated by Estrogen (E2) and Progesterone.
FSH and LH are directly responsible for the follicular development in the ovaries and formation and release of mature egg and indirectly responsible for the uterine cycle. The developing follicular cells secrete estrogen and progesterone that causes endometrial thickening. After ovulation, the follicle that releases the mature ovum becomes corpus luteum that continues to secrete E2 and Progesterone.
The release and changing levels of FSH and LH during the different phases of uterine cycle are key to the proper functioning of the female reproductive cycle.
So what triggers the release of these hormones and regulates their levels throughout the cycle?
It is the hypothalamus in the brain which acts as a hormonal master switch that runs the entire female reproductive cycle by releasing Gonadotropin Releasing Hormone (GnRH) which in turn stimulates the anterior pituitary to release FSH and LH.
FSH and LH is needed for a successful ovarian cycle which would in turn ensure production of Estrogen and Progesterone needed for the endometrium preparation in the uterine cycle.