(a) The menstrual cycle is the regular natural change that occurs in the female reproductive system (specifically the uterus and ovaries) that makes pregnancy possible. The cycle is required for the production of ovocytes, and for the preparation of the uterus for pregnancy.

At the beginning of menstrual cycle, the brain signals the pituitary gland to produce a hormone called the follicle-stimulating hormone (FSH), which in turn signals the ovaries to begin forming and maturing eggs for this cycle. As these follicles mature, the level of estrogen in the body rises, indicating that an egg is ready. This first half of the menstrual cycle is called the follicular phase.

*When high estrogen levels signal an egg is ready, the pituitary gland then produces a luteinizing hormone (LH), triggering the ovary to release the mature egg by the process of ovulation. The day ovulation happens is the first day of the second half of the cycle, the luteal phase.

During the luteal phase of the menstrual cycle the ovaries (the corpus luteum) begin to increase the levels of progesterone in the body. Progesterone, secreted by ovary regulates and maintains the inner lining of the uterus both during menstrual cycle. Progesterone is a hormone that prepares the lining of the uterus for pregnancy. If the egg is fertilized and implanted, the body continues to produce progesterone and if not, then progesterone levels fall, and that month's menstrual period begins.

(b) The follicular phase is known as the Proliferative phase because the primary follicle grows into the mature Graafian follicle and the endometrium of the uterus regenerates through proliferation simultaneously. The hormones LH and FSH stimulate follicular development. 

(c) The primary follicles in the ovary grow to become a fully mature Graafian follicle and simultaneously the endometrium of uterus regenerates through proliferation. These changes in the ovary and the uterus are induced by changes in the levels of

pituitary and ovarian hormones. The secretion of LH and FSH increases gradually during the follicular phase, and stimulates follicular development as well as secretion of estrogens by the growing follicles. The LH surge induces rupture
of Graafian follicle and thereby the release of ovum (ovulation). The
ovulation (ovulatory phase) is followed by the luteal phase during which
the remaining parts of the Graafian follicle transform as the corpus
luteum. The corpus luteum secretes large amounts of progesterone which is essential for maintenance of the endometrium.

(d)