Describe the phases of the ovarian cycle and the hormonal changes that occur during each phase.

The ovarian cycle has two phases: the follicular phase and the luteal phase. During the follicular phase, FSH stimulates follicle growth and estrogen secretion, and the dominant follicle is selected; high estrogen levels eventually cause an LH surge around day 14. The LH surge triggers ovulation and transforms the ruptured follicle into the corpus luteum. During the luteal phase, the corpus luteum secretes progesterone and some estrogens, which inhibit FSH and LH; if pregnancy does not occur, the corpus luteum degenerates and hormone levels fall, allowing the cycle to restart.

The follicular phase lasts from about day 1 to day 14 of the cycle. During this phase, FSH and LH stimulate vesicular follicles to grow and mature. FSH acts mainly on granulosa cells, causing them to release estrogens, while LH prompts thecal cells to release androgens that granulosa cells convert into estrogens. As estrogen levels rise, they exert negative feedback on the hypothalamus and anterior pituitary, and inhibin from granulosa cells also inhibits FSH release. Because of this, only the dominant follicle survives while the others deteriorate. When the dominant follicle produces a critical level of estrogen, estrogen briefly exerts positive feedback, leading to a sudden LH surge around midcycle. The LH surge makes the primary oocyte complete meiosis I and causes ovulation at about day 14. It also converts the ruptured follicle into the corpus luteum. The luteal phase runs from about day 14 to day 28. In this phase, LH stimulates the corpus luteum to secrete large amounts of progesterone and some estrogens. These ovarian hormones exert strong negative feedback on the hypothalamus and anterior pituitary, inhibiting FSH and LH release, which prevents new follicles from maturing and prevents further LH surges. If fertilization does not occur, the corpus luteum begins to degenerate in about 10 days, so estrogen and progesterone levels drop sharply around days 26 to 28, ending their blockade of FSH and LH secretion and starting the next cycle. The luteal phase is always about 14 days from ovulation to the end of the cycle, regardless of total cycle length.