Each cycle, the body works around the clock to prepare for a potential pregnancy, even if one doesn’t occur. This series will identify which hormones dominate different cycle phases and how they influence your mental and physical fitness. In turn we can then start to make smart training and lifestyle adaptations to make the most of being a woman.
The average cycle is 28 days… or is it?
A 28-day cycle with ovulation on day 14 is often used to represent the ‘average’ cycle, but the truth is that there is no one average cycle length. Only one in six women who menstruate experience a 28-day cycle. And ovulation on day 14 is a reality for even fewer people. The cycle lengths are mostly ranged between 21 and 35 days, without a single and universally applicable length for everybody. A range of 21 – 35 days is completely normal, and cycle length depends on many different factors, including an individual’s medical history.
Factors that affect menstrual cycle length
The key factor when it comes to cycle length variability is one we can’t do much about – our age. In terms of cycle length and variability, age plays a larger role than lifestyle and physical factors like the body mass index ( a measure of height and weight ). For example, it’s more common to have longer average cycles when you’re younger, with ovulation usually happening before day 14. Though less influential than age, lifestyle factors can still affect cycle length variability. High levels of stress and lack of exercise can contribute to shorter cycles for some. Smoking and drinking are a bit unusual in this regard. While we can’t say precisely how many cigarettes a day will start changing anybody’s cycle, we know that heavy smoking can affect a person’s body and influence their reproductive health.
A higher percentage of women with short cycles (20 days or less) reported experiencing high levels of stress on a regular basis and not exercising compared to women with normal and long cycles.
Surprisingly enough, habits such as alcohol consumption and smoking didn’t show any significant relation to menstrual cycle length. (2)
Focus on your own health, not abstract averages
All of this means that if your cycle is consistently longer or shorter than the ‘average’ 28 days… don’t worry! This metric doesn’t actually reflect the experiences of most women. Focus on your own average and what constitutes a typical cycle for you. By tracking your periods, you will be able to see how your cycle length varies over months and years and receive comprehensive reports that detail all of your particular metrics. And remember, if you worry about your cycle, always contact your health care provider. Self–diagnosing can be harmful.
The two phases of your menstrual cycle
Your menstrual cycle has two phases: the follicular phase and the luteal phase. The first day of menstruation marks the beginning of the follicular phase. During this phase, an egg matures in the ovary. Then ovulation occurs – the ovary releases this mature egg. Ovulation is a key process that occurs at the end of the follicular phase and actually splits your cycle into two phases.
After ovulation, the luteal phase begins. If at the end of this phase the released egg wasn’t fertilized, the uterine lining sheds, and your period begins.
The luteal phase places metabolic demands on the woman, and it appears more economical to regenerate and shed than to continuously sustain a fully secretory endometrium (Strassmann, 1996). The cost of menstrual bleeding is considered low, and thus bleeding may only be a side-effect of a blood volume that cannot be re-absorbed during endometrial regression (Strassmann, 1996). (1)
As already mentioned, normal cycle length can vary between 21 – 35 days with an average of 28 days. If you have a 28-day cycle, your follicular and luteal phases are roughly equal in length, around 14 days each.
Both the follicular and luteal phases produce different types of hormones and are accompanied by different types of symptoms.
What happens in the ovaries and uterus during each cycle?
The ovarian cycle consists of three phases, and so does the uterine cycle. Being aware of the processes occurring during each phase will help you understand your body better.
The first phase of the ovarian cycle is called the follicular phase. During this time, an egg matures in the ovary. The follicular phase coincides with two phases of the uterine cycle:
Menstruation phase: the endometrium ( uterine lining ) sheds, which results in bleeding ( your period ).
Proliferative phase: a new layer of endometrium develops to prepare for receiving a fertilized egg.
The follicular phase is followed by ovulation, which is when the follicle ruptures and an egg is released into the uterine tube. This is the second phase of the ovarian cycle called the ovulatory phase.
What is left of the ruptured follicle turns into a structure called the corpus luteum. This marks the beginning of the third phase in the ovary, the luteal phase.
The ovulatory and luteal phases of the ovarian cycle coincide with the final phase of the uterine cycle: the secretory phase, which is when the endometrium thickens, develops blood vessels and glands, and synthesizes nutrients for the implantation of a fertilized egg. If this doesn’t occur, you get your period, and the cycle begins again.
Which hormones control the menstrual cycle?
During each menstrual cycle, the same processes are supposed to happen in the body.
An egg matures, and the uterus prepares the endometrium ( the uterine lining ) for a possible pregnancy. If conception does not occur, hormonal changes trigger the endometrium to shed. If conception does occur, different hormonal changes take place so a pregnancy can develop.
But the effects of hormones don’t stop there. Changes in your mood, sex drive, skin, hair, and overall well-being are also influenced by changing hormone dynamics throughout the cycle. The effects that hormones have are the results of how they work together, more than how they work on their own. Let’s look at four important hormones that affect your body and cycle.
The two main female reproductive hormones are estrogen and progesterone.
Estrogen ( estradiol ) stimulates the growth of the egg follicle and regulates discharge and the thickness of the uterine lining. It also affects your brain ( including your mood ), bones, heart, skin, and other tissues.
Progesterone prepares the endometrium for potential pregnancy and, together with estrogen, is responsible for the release of an egg during ovulation. It also influences crucial processes in your body like metabolism, appetite, mood, and immune system function.
But there are two other key players in the menstrual cycle – follicle-stimulating hormone ( FSH ) and luteinizing hormone ( LH ).
Follicle–stimulating hormone helps an egg develop so it can be released from an ovary for ovulation.
Luteinizing hormone also supports the process of ovulation. It surges along with estrogen to trigger the release of the egg.
In part two we will discuss the luteal phase, the role of estrogen in the female body, and abdominal pain at different stages of the cycle. Keep an eye out for more information about the menstrual cycle next month!
(1) Strassmann, B.L., 1996. Energy economy in the evolution of menstruation. Evol. Anthropol. 5, 157–164.
(2) Grieger, Jessica A, and Robert J Norman. “Menstrual Cycle Length and Patterns in a Global Cohort of Women Using a Mobile Phone App: Retrospective Cohort Study.” Journal of Medical Internet Research, JMIR Publications Inc., Toronto, Canada, 24 June 2020, www.jmir.org/2020/6/e17109/.