What are the main phases of the menstrual cycle?
Knowing how each woman’s menstrual cycle works, as well as its different phases, helps us understand why there are specific days when a female is most fertile. Identifying those days makes it easier to fall pregnant naturally. Likewise, in assisted reproduction we take into consideration each woman’s cycle in order to plan for treatment.
Each phase of the menstrual cycle is regulated by hormones that make everything function. We’ll identify the important hormones in each stage. So take notes… because their names aren’t easy!
What is the menstrual cycle and what’s its purpose?
The menstrual cycle is the process by which the female reproductive system gets ready for a pregnancy by creating the ideal conditions needed for this to occur.
Although it may seem very straightforward, for all of this to happen the central nervous system (hypothalamus and pituitary gland), ovaries and uterus must be properly coordinated. All of this must be mediated and regulated through the interaction of different hormones:
- Follicle stimulating hormone (FSH), secreted by the pituitary gland.
- Luteinising hormone (LH), also secreted by the pituitary gland.
- Oestrogen, secreted by the ovaries.
- Progesterone, also secreted by the ovaries.
When a female’s hormone levels are normal and everything functions properly, the actions of the hormones in each phase will make it so that ovulation occurs correctly. Likewise, the inner lining of the uterus (the endometrium) will undergo changes that are necessary for the implantation of an embryo ‑ the result when an egg is fertilised by a spermatozoid in the outer part of a fallopian tube.
What are the phases of the menstrual cycle?
The menstrual cycle is made up of three main phases: menstruation, the follicular phase and the luteal phase. We could also talk about ovulation, which takes place between the follicular and luteal phases.
The entire cycle lasts 28 days on average, but it’s considered normal to see anywhere from 21 to 35 days between cycles. The first day of the menstrual cycle is the first day of menstruation or genital bleeding, and this is when we begin counting cycle days until the next period arrives.
But… what happens during each phase of the menstrual cycle and how long does each phase normally last? We’ll talk about that next.
Menstruation is the phase of genital bleeding which lasts for about the first 3 to 4 days of the cycle. The body of a woman during her reproductive years prepares itself each month for a possible pregnancy. If she does not get pregnant that month, the levels of the hormones oestrogen and progesterone produced by the ovaries will start to decline, which will cause the top layer of the endometrium -which had prepared itself to receive the embryo- to be shed or eliminated.
In this phase we may see different medical conditions such as dysmenorrhea, which are painful periods or menstruation that are most often caused by endometriosis; or hypermenorrhea, periods with an excessively heavy flow, etc. It is important for you to observe your cycles from month to month, because if your ovulation is not regular and these or other conditions are present it’s advisable that you see a specialist in assisted reproduction if you are trying to conceive.
The follicular or proliferative phase begins the days following menstruation. During this phase the ovaries get to work in order to produce a mature egg. How do they do it? A follicle which contains an egg will develop in the ovary and mature, resulting in correct ovulation.
This follicle, which we call the dominant follicle, will produce oestrogens, the main hormone responsible for growth of the superficial layer of the endometrium. The length of this stage may vary and is the reason why women’s menstrual cycles last for differing amounts of time, and why the length of the same woman’s cycles may vary from month to month.
Follicle development is mainly regulated by the secretion of FSH and LH by the pituitary gland. As this phase progresses, oestrogen levels grow increasingly higher. When it reaches the threshold, these high levels of oestrogen will stimulate the production of the LH surge from the pituitary gland, thus triggering the phenomenon known as ovulation.
This quick rise in LH levels leads to the final maturation of the egg, rupture of the follicle and, as a result, ovulation. In regular, 28‑day cycles this occurs on day 14. In some cycles the normal hormonal changes may occur but the follicle does not rupture, and in some cycles a dominant follicle does not develop: these are known as anovulatory cycles.
Following ovulation comes the secretory or luteal phase which is characterised by the production of progesterone by the corpus luteum in the ovary and the production of oestrogen as well, but in lower amounts. The corpus luteum is what remains of the ruptured follicle after ovulation.
The endometrium -which had grown and thickened thanks to stimulation by oestrogens in the uterus-, will mature during this phase as a result of stimulation by progesterone. This phase will last for 14 days in any ovulatory cycle. If no pregnancy takes place the corpus luteum will stop producing hormones, hormone levels will fall and menstruation will begin, thus indicating the start of a new menstrual cycle.
Other interesting facts about the menstrual cycle
A woman’s first menstrual cycle or first period is known as “menarche” and takes place between the ages of ten and sixteen, the average being twelve and a half years old. This average may vary from one country to another depending on socioeconomic status and hereditary factors.
The last menstruation a woman will have in her lifetime is known as “menopause”. It marks the permanent end of a female’s reproductive years and takes place, on average, at the age of fifty or fifty‑one. When menopause occurs before the age of forty it is considered premature menopause.
The absence of menstruations or periods is called “amenorrhea”. It could be primary amenorrhea, which is when menstruation has not begun by the time a female turns sixteen, or secondary amenorrhea, which is the absence of periods for three or more months in a female who previously had menstrual cycles. The causes of amenorrhea can be anatomical, hormonal, genetic, etc. During pregnancy there is an absence of periods or menstruation, known as physiologic amenorrhea.
As you can see, having a good understanding of the different phases that make up your menstrual cycle will help you not only determine which days are your most fertile days, but also comprehend the steps that must take place in your body in order to conceive.
If you notice that your menstrual cycle is not normal or you detect complications in some of the phases of your cycle, track your cycle for some time and, if the problem persists don’t hesitate to visit a fertility specialist as soon as possible. At our fertility clinic in Spain we see cases like this quite often. Contact us whenever you’d like if you have questions, and we’ll be happy to help you.
