The menstrual cycle is the monthly process of preparing your eggs and womb for a possible pregnancy.
This article is pending medical review.
Written by Carolin Becker
Reviewed by Lea Dörner, Miriam Babukhian and Alessandra Papitto
Edited by Juliëtte Gossens
Menstruation, also known as period, is part of the reproductive cycle during the fertile years of most women and people with ovaries and a uterus. It’s a sign that the body’s reproductive system is functioning properly. The first menstruation usually occurs during puberty (which typically starts between 8-15 years) and continues until menopause (usually in your late 40s to early 50s). The menopause marks the natural end of fertility. During menstruation, the lining of the uterus (the endometrium) is shed and expelled from the body through the vagina. This typically occurs about every 28 days (though different people’s cycle lengths vary widely) and can last anywhere from a few days to a week (1, 2).
For more background information, you can read our article on the biology of egg cells.
What we're covering
Why do we menstruate?
Menstruation is an important topic in many people’s lives. Even though there is still a lot of stigma surrounding the topic, we’ve seen a more open discussion over the past few years. For example, there have been political discussions on menstrual leave or the taxation of period products in many different places.
But why do we even menstruate? That's a good question, but science does not have a clear answer yet. In the past, one of the theories was that menstruation is a defense mechanism against pathogens that come into the uterus (womb) together with sperm during sex. More recently, it’s thought that it’s an evolutionary consequence of the way a human pregnancy develops. A baby’s placenta is very tightly associated with the uterus of the pregnant person. But it shouldn’t grow too far into the uterus, because that could damage the uterus so future pregnancies aren’t possible anymore. To prevent this, the uterus builds up a protective lining each month that an embryo can grow into together with its placenta, as a sort of cushion. This is called “decidualization”. That way, the embryo and placenta won’t grow directly into the wall of the uterus. But if you don’t become pregnant, that lining isn’t needed, so it’s shed and rebuilt the next month (3).
Though scientists are not completely sure why menstruation developed, what is for sure is that the menstrual cycle in humans is caused by the rise and fall of several sex steroid hormones. Those hormones lead to the regular shedding and rebuilding of the lining the uterus (the endometrium) (1).
The reproductive cycle
The reproductive cycle varies from person to person. The length of the cycle, of which menstruation is a part, can last anywhere between 21 to 35 days. That’s considered a normal cycle length, though some people’s cycles are shorter or longer (2).
Usually, menstruation occurs between the ages of 10 to 55, but again, this is very individual. Variations between cycles are normal and might be due to different factors, such as life circumstances, birth control or stress (1). More on the possible reasons for why you’re not menstruating can be found here.
Information on what is happening inside our bodies during menstruation can sometimes be confusing. That is mainly due to the fact that the reproductive cycle consists of 2 different cycles happening simultaneously, both preparing a menstruating person to become pregnant and to sustain the embryo throughout pregnancy (2):
The ovarian cycle: this cycle prepares the hormone-producing tissue (endocrine tissue) of the ovaries and is responsible for the preparation and release of an egg cell
The menstrual cycle (also called "uterine" cycle): this cycle is responsible for the preparation and maintenance of the uterine lining (the endometrium).
Both cycles start on day 1, which is the first day of your menstruation.
When reading the following, you should keep in mind that phases may vary in length between and within menstruating people. The fertile period (or fertile window) typically also varies in this way.
So let's start with the ovarian cycle, which can be split up into 3 parts: the follicular phase, the ovulation, and the luteal phase.
The ovarian cycle
Follicular phase (Day 1-14)
The ovarian cycle starts with the follicular phase. In a 28-day cycle, this typically lasts for about 14 days. During that phase, follicles start to grow at the surface of the ovary and prepare the eggs inside them for ovulation (1, 2). We are born with around 4 million oocytes (immature eggs) and during each cycle, 3-11 follicles with egg cells start maturing. However, only one of these eggs will mature all the way. This egg is located in the so-called "dominant follicle”. You can read more about the maturation of follicles here.
The follicle growth is stimulated by the rise of the hormones FSH (follicle-stimulating hormone) and LH (luteinizing hormone). Scientists think that the dominant follicle is the one that’s most sensitive to FSH, which is why it can grow the largest. Both of these hormones are released by the anterior pituitary gland in the brain. The growing follicles start releasing estrogens (1, 2, 4).
It’s the rise in these hormones that induces the next phase: ovulation.
Ovulation (Day 14)
Ovulation typically happens on day 14. On this day, the concentration of LH is the highest as a result of high estrogen levels. This causes the dominant follicle to rupture and release the mature egg. Now, the egg is ready to be fertilized and travels down the fallopian tube. The other follicles and eggs that were growing will be reabsorbed by the ovary and broken down (2).
Because sperm cells can survive for about 5 days after sex, the so-called “fertile window” starts 5 days before ovulation and ends on the day of ovulation. You can only become pregnant within this fertile window. That seems like great birth control, but it’s very difficult to estimate exactly when you’ll ovulate. That’s because the length of the follicular phase in your cycle often differs from month to month (1). If you want to read a bit more about using knowledge of your cycle as birth control, you can go here.
Luteal phase (Day 15-28)
During the last phase of the ovarian cycle, the cells of the ruptured dominant follicle transform into the corpus luteum (the “yellow body”). This structure prevents the production of FSH and LH by releasing estrogen and progesterone. These hormones are important for the development of the endometrium, as you will see in the menstrual cycle below.
In case no fertilization occurs during this time, the corpus luteum degenerates. The decrease in estrogen and progesterone causes a rise in FSH, which triggers a new cycle. This starts again with the follicular phase (2).
Now we will have a look at the menstrual cycle. This also has three parts: the menstruation, the proliferation phase, and the secretion phase.
The menstrual cycle
Menstrual flow (Day 1-7)
As we’ve explained above, the corpeus luteum degenerates if no egg was fertilized during the past cycle. Because the hormones produced by the corpeus luteum kept the endometrium (the womb lining) intact, the loss of the corpeus luteum leads to the shedding of the endometrium. This is what a period is: the lack of hormones causes the blood flow to the endometrium to stop. That damages the endometrium so it detaches from the wall of the uterus. The wall of the uterus is very muscular, and contractions of this muscle pushes the bits of endometrium through the cervix and out of the vagina. The tissue can also be pushed through the fallopian tubes into the abdomen. These contractions also make sure that the blood vessels in the wall of the uterus, that bleed as the endometrium detaches, are pressed closed. The muscle contractions are what you experience as cramping during your period.
Proliferative phase (Day 8-14)
The second half of the follicular phase is then accompanied by the regrowth (proliferation) of the endometrium, after it was shed during menstruation. This proliferation is caused by rising levels of estrogen. It prepares the uterus for implantation of a fertilized egg into the endometrium. Right after menstruation, the endometrium is only 2 millimeters thick (nearly 0.1 inch). During the proliferative phase, it grows until it’s about 12 millimeters thick (about half an inch). That’s about as thick as it will get (2, 4).
Secretory phase (Day 15-28)
The secretory phase happens at the same time as the luteal phase. During this phase, the endometrium is working hard. There are glandular cells in the endometrium. These cells produce lots of energy-rich mucus (slime) that they secrete into the endometrium. It makes the uterine lining soft and pillowy, and provides nutrients: perfect for any fertilized egg that wants to implant and grow. In case the egg released during ovulation is not fertilized and there is no implantation, the corpus luteum breaks down over time. As we’ve seen, the corpus luteum (and the hormones it produces) is important to keep the endometrium intact. So, the decrease in progesterone (which is produced by the corpus luteum) causes the endometrium to break down as well. This leads directly into menstruation (2).
To give you a short summary of what’s happened (because it’s a lot): over the course of about 28 days, one follicle from one of your ovaries grows largest (the dominant follicle). This follicle then goes through ovulation, at which point it releases the egg inside it. At the same time, you’re having your period during which your uterine lining (endometrium) is shed, and then it’s built back up. After ovulation, the released egg is looking to get fertilized and the other cells from the follicle (the corpus luteum) produce hormones to keep the endometrium intact. At the same time, cells from the endometrium are producing slime and nutrients for when the egg is fertilized and wants to implant and grow. If the egg didn’t get fertilized, the corpus luteum breaks down, and the endometrium does too. You’ll have your period again, and the whole cycle starts anew. That’s it!
All of these changes and phases a menstruating person goes through during their cycle are caused by the rise and fall of several hormones. How long that cycle is, and how long the different phases within it are, differs per person – and even within the same person. That’s also why your fertile window is difficult to predict exactly (1).
Finally, there is more to menstruation than the blood flow. Your menstrual pattern – especially sudden changes in it - can be an indicator for your health. That’s why it’s so important that people feel comfortable talking about their periods and seeking out the resources and support they need to manage their reproductive health.
Mihm M, Gangooly S, Muttukrishna S. The normal menstrual cycle in women. Animal Reproduction Science. 2011;124(3-4):229-36. DOI: 10.1016/j.anireprosci.2010.08.030
Hoffman BL, Schorge JO, Halvorson LM, Hamid CA, Corton MM, Schaffer JI (eds.). Williams Gynecology. 4th ed. New York: McGraw-Hill Medical; 2020.
Jarrell J. The significance and evolution of menstruation. Best Practice & Research Clinical Obstetrics & Gynaecology. 2018;50:18-26. DOI: 10.1016/j.bpobgyn.2018.01.007
Messinis IE, Messini CI, Dafopoulos K. Novel aspects of the endocrinology of the menstrual cycle. Reproductive Biomedicine Online. 2014;28(6):714-22. DOI: 10.1016/j.rbmo.2014.02.003
Please note: the information we provide to you here is for educational purposes only. If you’re experiencing any discomfort or have any complaints or questions about your health, please contact your doctor or other relevant health professional. We don’t provide medical advice.