Animal Biology: The Menstrual Cycle

The menstrual cycle is central to human reproduction and it is controlled by certain hormones in females of reproductive age.
The entire cycle lasts for around 28 days, although there can be some variation between individuals.
The cycle commences with menstruation, the process in which the uterus sheds its lining. This is commonly referred to as a ‘period’ and usually lasts between 3 to 7 days.
After menstruation, the Follicular phase begins. This is where the Follicle Stimulating Hormone (FSH) is released by the pituitary gland. FSH stimulates the growth of around 5 to 20 follicles on the ovary, each containing an immature ovum.
High levels of FSH also stimulate the production of oestrogen. This hormone helps in rebuilding the wall of the uterus that was shed during menstruation.
By around day 14, levels of oestrogen peak and trigger the release of Luteinising Hormone (LH). This event, often called the LH surge, results in the matured ovary releasing an ovum. This is called ovulation.
Post ovulation, the Luteal phase begins. Progesterone is produced during this phase, preventing the release of further eggs and maintaining the lining of the uterus for potential pregnancy.
If no fertilisation occurs, levels of progesterone drop, the uterine lining is shed, marking the start of another menstrual cycle.
When a female reaches menopause, usually around the age of 50, the menstrual cycle halts. This is due to the ovaries no longer releasing eggs and the production of oestrogen and progesterone decrease.
Understanding the menstrual cycle is key to concepts such as contraception, fertility treatments, and understanding reproductive health disorders like polycystic ovary syndrome (PCOS) and endometriosis.