The different hormonal and physical processes driving your menstrual cycle. 
The menstrual cycle involves the monthly release of an egg from one of the ovaries, a process called ovulation. The uterus prepares to receive a fertilized egg via a series of hormonal changes. If the egg does not get fertilized by a sperm cell then the lining of the uterus isn’t needed and it is shed through the vagina – resulting in a menstrual period. 
This menstrual cycle is driven by changes in hormones, particularly estrogen, progesterone, luteinizing hormone (LH) and follicle-stimulating hormone (FSH). Estrogen and progesterone are called ‘sex hormones’ because they are secreted by your ovaries, which are the female sex organs. LH and FSH are gonadotropins because they are secreted from cells in the anterior pituitary called gonadotrophs (gonads are the formal term for the sex organs – ovaries in females, and testes in males).
There are three stages in the menstrual cycle: the follicular stage (before release of the egg); the ovulatory stage (egg release); and the luteal stage (after egg release).
The first day of menstrual bleeding marks the first day of the follicular phase. At this time, levels of estrogen and progesterone are low and the top layers of the thickened lining of the uterus (endometrium) break down and shed, creating the menstrual bleeding. At the same time, follicle-stimulating hormone (FSH) levels increase and cause several follicles, each containing an egg, to develop in the ovaries. Then the FSH levels start to decrease and only one follicle continues to develop and starts to produce estrogen.
This becomes the ovulatory phase when luteinizing hormone (LH) and FSH both rapidly increase. LH causes the egg to be released from the follicle (ovulation). Estrogen levels decrease when LH and FSH levels are heightened. Progesterone levels begin to increase.
In the luteal phase both LH and FSH levels decrease. A structure called the corpus luteum forms from the ruptured follicle (that just released the egg) and it begins to produce progesterone. Estrogen levels stay elevated for much of the luteal phase. The presence of progesterone and estrogen cause the uterine lining to thicken in preparation for the implantation of a fertilized egg.
If the egg isn’t fertilized, the corpus luteum breaks down and stops producing progesterone. Estrogen levels also decrease. This causes the top layers of the uterine lining to break down and begin shedding, starting menstrual bleeding and the beginning of a new menstrual cycle.  
Among women in the early menopause transition, abnormalities in timing of menstrual bleeding – both changes in the length of your cycle or changes in the number of days you typically bleed – are the result of changes in hormone levels. These changing hormones also result in anovulation, cycles where you do not ovulate, or release an egg.
Abnormally heavy periods, however, do not appear to have a hormonal basis. Rather, heavy periods are associated with obesity (a body mass index (BMI) of 30 and above) and the presence of benign fibroids (leiomyomata).  Heavy periods can be identified by:
- needing to change your pad or tampon every hour;
- bleeding that goes on for more than seven days;
- bleeding (not spotting, but proper menstrual flow) happens more frequently than every 3 weeks; and/or
- have a menstrual flow that contains blood clots larger than the size of a quarter.
Heavy periods are less likely after anovulatory cycles where no egg was released.