What is called menstrual cycle ? Describe its phases ?

The reproductive cycle in the female primates (eg. monkeys, apes and human beings) is called menstrual cycle.

The first menstruation begins at puberty and is called menarche.

In human females, menstruation is repeated at an average interval of about $28 / 29$ days and the cycle of events starting from one menstruation till the next one is called the menstrual cycle.

One ovum is released (ovulation) during the middle of each menstrual cycle.

Menstrual cycle or events of uterus cycle are cyclic changes occurring in endometrium.

Female sex hormones produced in blood are estrogen and projesterone, are responsible in their layer.

The events of this cycle can be divided in $28$ days in following $3$ phases.

Menstrual phase (Day $ 1-5$ ) : The menstrual flow results due to breakdown of endrometrial lining of the uterus and its blood vessels which forms liquid that comes out through vagina. It lasts for $3-5$ days.

$50 \mathrm{ml}$ to $150 \mathrm{ml}$ blood is wasted during this time. This phase is also called menstrual phase. Lack of menstruation may be indicative of pregnancy.

However it may also be caused due to some other underlying causes like stress, poor health etc.

Follicular phase (day $ 6-14$ ) : During this phase 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 level of pituitary and ovarian hormones.

The secretion of gonadotropins ($ LH$  and $ FSH$ ) increases gradually during the follicular phase and stimulates follicular development as well as secretion of estrogens by the growing follicles.

In the middle of cycle (about $14^{\text {th }}$ day) both $ LH$  and $ FSH$  attain a peak level.