Phases of Ovarian cycle

Follicular Phase

During this phase, the uterine lining thickens in preparation for a potential pregnancy, and an egg matures in the ovaries. This phase starts on the first day of menstruation and ends with ovulation.

FSH stimulates primordial follicles to mature into Graafian follicles and promotes the production of Oestrogen and inhibin within the ovary. Studies indicate that most of the estrogen is produced by the dominant follicle.

The dominant follicle continues to mature, growing approximately 2 mm/day until it reaches a diameter of 18 to 29 mm . FSH also induces the development of LH receptors within the dominant follicle, preparing it for the next step—ovulation.

Ovulatory phase

Approximately midway through the cycle (around day 14 in a 28-day cycle), the mature egg is released from the ovary into the fallopian tube, ready for fertilization. 

At this stage Oestrogen causes a brief positive feedback to the Hypothalamus and Anterior pituitary gland resulting to LH surge with a smaller rises in FSH concentrations.

Luteal Phase

Following ovulation, the ruptured follicle transforms into the corpus luteum, which produces progesterone to further prepare the uterine lining for a potential pregnancy. 

The dominant hormone during this phase is progesterone, which is stimulated by LH. Progesterone stimulates the maturation of the endometrium in preparation for the potential implantation of a fertilized ovum.

If fertilization doesn’t occur, the corpus luteum degenerates, and the cycle starts anew with menstruation. 

Menstruation

The abrupt decline in progesterone and estradiol levels at the end of the luteal phase triggers the shedding of the endometrium, which can no longer be sustained without these hormones. This shedding is referred to as menses. The first day of menstrual bleeding is designated as day 1 of the menstrual cycle, meaning menstruation occurs during the initial days of the follicular phase.