In this post, we will cover everything you need to know about your cycle.
Key points covered:
- The menstrual cycle starts with the first day of the period and ends when the next period begins
- The brain, ovaries, and uterus work together and communicate through hormones (chemical signals sent through the blood from one part of the body to another) to keep the cycle going.
- The first part of the cycle (follicular phase) prepares an egg to be released from the ovary and builds the lining of the uterus
- The second part of the cycle (luteal phase) prepares the uterus and body to either accept a fertilised egg or to start the next cycle if pregnancy doesn’t occur
- The menstrual cycle is more than just the period. In fact, periods are just the first part of your cycle.
- Understanding the menstrual cycle is important because it can impact the body from head to toe. You can see changes in your hair skin, bowel movements, mental health, physical strength, libido and so on.
- Hormonal methods of birth control prevent some or all of the steps in the cycle from happening.
Sections in the article:
- What is the menstrual cycle?
- How long is the menstrual cycle?
- How many women have regular cycles?
- What are the menstrual cycle phases?
- What hormones are involved in the menstrual cycle?
- Breakdown of hormones and their functions in the menstrual cycle
- Follicle-stimulating hormone
- Luteinising stimulating hormone
What is the menstrual cycle?
By definition, the menstrual cycle begins with the first day of your period, which is counted as ‘Day 1’ and ends just before the next period (bleeding).
It’s the female body’s way of preparing for a possible pregnancy every month.
Understanding your cycle is very important as it impacts every aspect of female wellbeing. As the hormones in your cycle change throughout the month, your body and mind also go through a number of changes.
How long is the menstrual cycle?
Menstrual cycles normally range from about 25 to 36 days. However, they can also be longer or shorter, in which case we have an ‘irregular cycle’.
How many women have regular cycles?
Only 10 to 15% of women have cycles that are exactly 28 days, and in at least 20% of women, cycles are irregular.
Having an irregular cycle means that their cycle is either longer or shorter than the normal range of between 25 to 36 days.
Usually, cycles are the most irregular in the years immediately after a woman starts to menstruate and before menopause.
What are the menstrual cycle phases?
There are 5 core phases in your cycle:
Phase 1: The Follicular:
When: Technically the follicular phase starts on the first day of the period until Ovulation.
What happens: After the period, the uterine lining builds back up again (aka the proliferative phase). During this phase, oestrogen rises as an egg prepares to be released.
The Follicular phase is characterised by two phases, Menstruation and Proliferative phase.
Phase 1.1: Menstruation (aka period):
When: Day 1 of your period until you stop bleeding
What happens: During this phase of your cycle, you shed your uterine lining (bleeding) and your oestrogen and progesterone levels are low.
Phase 1.2: Proliferative phase
When: Day after your period has ended up until ovulation
What happens: The uterus builds up a thick inner lining. While the ovaries are working on developing the egg-containing follicles, the uterus is responding to the estrogen produced by the follicles, rebuilding the lining that was just shed during the last period. This is called the proliferative phase because the endometrium (the lining of the uterus) becomes thicker.
Phase 3: Ovulation:
When: About halfway through the cycle, around day 13-15 (but this can change cycle-to-cycle and you might even have the occasional cycle where you don’t ovulate at all).
What happens: The release of the egg from the ovary, mid-cycle. Oestrogen peaks just beforehand, and then drops shortly afterwards.
The dominant follicle in the ovary produces more and more estrogen as it grows larger. The dominant follicle reaches about 2 cm (0.8 in)—but can be up to 3 cm—at its largest right before ovulation (6,7). When estrogen levels are high enough, they signal to the brain causing a dramatic increase in luteinizing hormone (LH) (11). This spike is what causes ovulation (release of the egg from the ovary) to occur. Ovulation usually happens about 13-15 days before the start of the next period (12).
Phase 4: Luteal Phase:
When: The time after ovulation and before the start of menstruation. It normally lasts between 14 to 16 days.
What happens: The body prepares for a possible pregnancy.
Once ovulation occurs, the follicle that contained the egg transforms into a corpus luteum and begins to produce progesterone as well as oestrogen, with progesterone levels peaking about halfway through the luteal phase.
If an egg is fertilised, progesterone supports the early pregnancy, alternatively, the uterine lining starts to break down resulting in menstruation.
What hormones are involved in the menstrual cycle?
The menstrual cycle is regulated by a number of different hormones. But the main ones are:
- Follicle-stimulating hormone (FSH): Stimulates egg development and the release of oestrogen.
- Luteinising hormone (LH): Stimulates the release of the egg (called ovulation). Stimulates oestrogen and progesterone production.
- Oestrogen: causes growth of the uterine lining. Inhibits FSH. Stimulates the release of LH and hence release of the egg. Inhibits LH after ovulation.
- Progesterone: maintains the uterine lining. Inhibits LH after ovulation.
Breakdown of hormones and their functions in the menstrual cycle
Follicle-stimulating hormone is produced by the pituitary gland. It regulates the functions of both the ovaries and testes. Lack or insufficiency of it can cause infertility in both men and women.
What is follicle stimulating hormone?
Follicle-stimulating hormone is one of the gonadotropic hormones, the other being the luteinising hormone, and both are released by the pituitary gland into the bloodstream.
Follicle-stimulating hormone is essential to pubertal development. In women, this hormone stimulates the growth of ovarian follicles in the ovary before the release of an egg and it also increases a type of oestrogen.
What is Oestrogen
Oestrogen is one of the main female sex hormones. While both women and men produce oestrogen, it plays a bigger role in women’s bodies.
It has many roles in the body, from controlling puberty to strengthening bones. Having too much or too little oestrogen can cause a range of different medical conditions.
There are three oestrogens – oestrone, oestradiol and oestriol – the most potent of which is oestradiol.
- Oestradiol is produced in women of childbearing age, mostly by the ovaries.
- Oestriol is the main oestrogen produced during pregnancy, mostly in the placenta.
- Oestrone, produced by the adrenal glands and fatty tissue, is the only type of oestrogen produced after menopause.
What is oestradiol?
Oestradiol is a steroid hormone made from cholesterol and is the strongest of the three naturally produced oestrogens.
It is the main oestrogen found in women and has many functions, although it mainly acts to mature and maintain the female reproductive system. A natural increase in blood oestradiol concentrations during the menstrual cycle causes an egg to mature and be released.
Another important role of oestradiol is to thicken the lining of the uterus so that the egg can implant if it becomes fertilised.
Oestradiol also promotes the development of breast tissue and increases both bone and cartilage density.
In premenopausal women, oestradiol is mostly made by the ovaries. Oestradiol levels vary throughout the monthly menstrual cycle, being highest at ovulation and lowest at menstruation. Oestradiol levels in women reduce slowly with age, with a large decrease occurring at menopause when the ovaries ‘switch off’. In pregnant women, the placenta also produces a lot of oestradiol especially towards the end of the pregnancy.
Men also produce oestradiol; however, the amounts produced are much lower than in women. Within the testes, some testosterone is changed into oestradiol and this oestradiol is essential for the production of sperm. In both sexes, oestradiol is also made in much smaller amounts by fat tissue, the brain and the walls of blood vessels.
What is luteinising hormone?
Luteinising hormone, like follicle-stimulating hormone, is a gonadotrophic hormone produced and released by cells in the anterior pituitary gland.
It is crucial in regulating the function of the testes in men and ovaries in women.
In men, the luteinising hormone stimulates Leydig cells in the testes to produce testosterone, which acts locally to support sperm production. Testosterone also exerts effects all around the body to generate male characteristics such as increased muscle mass, enlargement of the larynx to generate a deep voice, and the growth of facial and body hair.
In women, luteinising hormone carries out different roles in the two halves of the menstrual cycle. In weeks one to two of the cycle, luteinising hormone is required to stimulate the ovarian follicles in the ovary to produce the female sex hormone, oestradiol.
Around day 14 of the cycle, a surge in luteinising hormone levels causes the ovarian follicle to tear and release a mature oocyte (egg) from the ovary, a process called ovulation. For the remainder of the cycle (weeks three to four), the remnants of the ovarian follicle form a corpus luteum.
The luteinising hormone stimulates the corpus luteum to produce progesterone, which is required to support the early stages of pregnancy if fertilisation occurs.
What is progesterone?
Progesterone belongs to a group of steroid hormones called progestogens. It is mainly secreted by the corpus luteum in the ovary during the second half of the menstrual cycle. It plays important roles in the menstrual cycle and in maintaining the early stages of pregnancy.
During the menstrual cycle, when an egg is released from the ovary at ovulation (approximately day 14), the remnants of the ovarian follicle that enclosed the developing egg form a structure called the corpus luteum.
This releases progesterone and, to a lesser extent, oestradiol. The progesterone prepares the body for pregnancy in the event that the released egg is fertilised. If the egg is not fertilised, the corpus luteum breaks down, the production of progesterone falls and a new menstrual cycle begins.
If the egg is fertilised, progesterone stimulates the growth of blood vessels that supply the lining of the womb (endometrium) and stimulates glands in the endometrium to secrete nutrients that nourish the early embryo.
Progesterone then prepares the tissue lining of the uterus to allow the fertilised egg to implant and helps to maintain the endometrium throughout pregnancy. During the early stages of pregnancy, progesterone is still produced by the corpus luteum and is essential for supporting the pregnancy and establishing the placenta.
Once the placenta is established, it then takes over progesterone production at around week 8-12 of pregnancy. During pregnancy, progesterone plays an important role in the development of the foetus; it stimulates the growth of maternal breast tissue; prevents lactation; and strengthens the pelvic wall muscles in preparation for labour.
The level of progesterone in the body steadily rises throughout pregnancy until labour occurs and the baby is born.
Although the corpus luteum in the ovaries is the major site of progesterone production in humans, progesterone is also produced in smaller quantities by the ovaries themselves, the adrenal glands and, during pregnancy, the placenta.
What is testosterone?
Testosterone is produced by the gonads (by the Leydig cells in testes in men and by the ovaries in women), although small quantities are also produced by the adrenal glands in both sexes.
It is an androgen, meaning that it stimulates the development of male characteristics.
Present in much greater levels in men than women, testosterone initiates the development of the male internal and external reproductive organs during foetal development and is essential for the production of sperm in adult life.
This hormone also signals the body to make new blood cells, ensures that muscles and bones stay strong during and after puberty and enhances libido both in men and women.
Testosterone is linked to many of the changes seen in boys during puberty (including an increase in height, body and pubic hair growth, enlargement of the penis, testes and prostate gland, and changes in sexual and aggressive behaviour). It also regulates the secretion of luteinising hormone and follicle stimulating hormone.
To effect these changes, testosterone is often converted into another androgen called dihydrotestosterone.
In women, testosterone is produced by the ovaries and adrenal glands. The majority of testosterone produced in the ovary is converted to the principle of female sex hormone, oestradiol.
- Fraser IS, Critchley HO, Broder M, Munro MG. The FIGO recommendations on terminologies and definitions for normal and abnormal uterine bleeding. Semin Reprod Med. 2011;29(5):383-90.
- Munster K, Schmidt L, Helm P. Length and variation in the menstrual cycle-a cross-sectional study from a Danish county. BJOG. 1992;99(5): 422–9.
- Treloar AE, Boynton RE, Behn BG, Brown BW. Variation of the human menstrual cycle through reproductive life. Int J Fertil. 1967;12(1 Pt 2):77-126.
- Fehring RJ, Schneider M, Raviele K. Variability in the phases of the menstrual cycle. JOGNN. 2006;35: 376-384.