The Science Behind the Menstrual Cycle: Hormonal Fluctuations and Phases
By Dr. Anita Mitra (The Gynae Geek)
If you’re someone who has chosen to read this article about the menstrual cycle, the chances are you’ve heard of the term ‘balancing your hormones’. Well, let me tell you, there is no such thing! I want to strip things right back to the basics before we take a deep dive into the finer details.
The first thing I need to do is clarify what we mean by the word ‘hormones’; they are the chemical messengers of the body, that are produced in multiple locations such as the brain, pancreas, ovary, and even the adipose (fat) tissue and provide a communication network between the different organs and tissues of the body to ensure they work in collaboration with one another. When it comes to female hormones, the ones that you’ve probably heard of are oestrogen and progesterone, as well as maybe luteinising hormone (LH), which is the hormone tested for by ovulation sticks, and follicle stimulating hormone (FSH) which can be checked in some situations to determine whether you’re in the menopause.
Oestrogen and progesterone are both made predominantly by the ovaries, but also by the adrenal glands. Oestrogen can also be made by the adipose tissue, and progesterone, which is the predominant hormone of early pregnancy is made in huge amounts by the placenta.
Now, the reason that I say hormones are never actually ‘balanced’ is because their production is based around the dynamics of other hormones, with the rise or fall of other hormones promoting an increase or decrease in production of another.
The menstrual cycle is an excellent example this. A common question that I ask patients in clinic is ‘how long is your menstrual cycle?’, and many people think this question means ‘how long is your period?’, so if you also thought that, you’re not alone! The menstrual cycle is the entire time from the start of one period until the next. The textbook duration is 28 days, but actually only about 13% of people have a ’28-day cycle’ (1,2). Anything between 21 to 35 days is ‘normal’, and if your cycle is within this range, you can generally assume that you’re ovulating. Cycles that are shorter or longer than this, are typically ‘anovulatory’, i.e. you aren’t ovulating, because it’s the act of ovulating that sets the time of your next period. And in order to ovulate, your hormones need to be ‘perfectly imbalanced’ to trigger the correct hormonal signals for a regular menstrual cycle.
The follicular phase
Day 1 of your period, the first day of proper bleeding, is the first day of the follicular phase of the cycle. The average length of a period is about 5 days, and a range between 3-8 days is considered normal. At this point in time, oestrogen, progesterone, LH and FSH levels are all at their lowest point and it’s these low levels that stimulate the pituitary gland in the brain to start producing FSH, which does what it says on the tin: stimulates follicles. Follicles are small little sacs inside the ovary that contain immature eggs. Every month several follicles are chosen to grow and mature in anticipation that they will be released during ovulation. They also produce oestrogen, which helps to thicken up the lining the uterus, which is called the endometrium. After your period, the endometrium is at its thinnest, and it needs to be rebuilt, thicken up and develop a network of blood vessels to make it suitable for implantation of a fertilized egg should there be one later during the menstrual cycle. This early phase of the menstrual cycle can also be described as a ‘proliferative phase’ because this growth of the endometrium is termed ‘proliferation’. Around about day 5-7, one of the follicles becomes ‘the chosen one’ and continues to grow whilst the other growing follicles shrink away. Oestrogen levels are still rising due its production by the ever-growing dominant follicle, and eventually reach their peak around day 12, which signals to the brain that it’s time to ovulate. In response, the brain releases a huge surge of luteinising hormone (LH) is required. I always describe LH to my patients as a big pin that comes along and pops the egg out of its sac, because about 24–36 hours after the start of the LH surge ovulation occurs. If you’ve ever used an ovulation prediction kit (OPK), LH is the hormone that’s being detected by them.
The luteal phase
Ovulation, which happens around day 14 in the textbook 28-day cycle, marks the start of the luteal phase. You can only ovulate on one occasion per menstrual cycle, but it is possible to release more than one egg at a time. Should each egg become fertilised, this produces non-identical twins (or triplets!). You’re more likely to have non-identical twins the older you get because your body makes more FSH to stimulate your ovaries as your egg reserve starts to fall.
After ovulation, a structure called the corpus luteum is left behind in the ovary, where is where the name ‘luteal phase’ comes from. This corpus luteum is responsible for producing progesterone, which is the ‘pro-gestational’ or ‘pro-pregnancy’ hormone. It prevents further thickening of the endometrium and instead ensures that it continues to enhance its blood supply so that it’s ready to house a potential pregnancy. Progesterone levels reach their highest levels 7 days after ovulation, and if there is a pregnancy it will produce human chorionic gonadotrophin (HCG), which is the hormone detected by urine pregnancy tests. HCG signals to the corpus luteum to continue producing progesterone to maintain the pregnancy until the placenta is big enough to produce sufficient HCG to take over. If there isn’t any HCG around, the corpus luteum starts to shrink away, causing progesterone levels to fall. The corpus luteum also produces oestrogen, and there is a second rise and fall in oestrogen levels that mirror that of the progesterone peak. As a result of the falling oestrogen and progesterone, the endometrium can no longer be maintained and the upper two-thirds fall away within the next 7 days, which is your period. Your body also produces prostaglandins in response to falling progesterone levels, and these are chemical messengers that stimulate uterine contractions to help the body to push out the shedding lining. It’s these contractions that are responsible for the period pains that many of us know only too well.
The duration of the luteal phase is fairly consistent due to a very fixed lifespan of the corpus luteum, with variation in cycle length arising from variations in the duration of the follicular phase. A small percentage of people may have an abnormally short luteal phase (usually less than 10 days) which is thought to occur as a result of reduce progesterone production by the corpus luteum, or an abnormal response of the endometrium to progesterone. It may be associated with difficulties in getting pregnant, but has also been described in fertile individuals with normal menstrual cycles (3). There are numerous scenarios that can such as extreme exercise, significant weight loss, obesity, stress, and uncontrolled thyroid abnormalities. In all of these scenarios there is an alteration of levels of multiple other hormones which ultimately result in the brain altering its signals to your ovaries.
And this brings me back to one of my very first statements: your hormones are all talking to one another. They need to work together, and abnormal levels of seemingly unrelated hormones, can result in changes in the menstrual cycle.
Polycystic ovarian syndrome (PCOS) is probably the most well-known condition affecting menstrual cycles, and it is in fact the most common female hormonal condition during the reproductive years. About 13% of women worldwide have PCOS, but it may be up to 27% depending on ethnicity and geographical location (4). If you have PCOS, periods are either completely absent, or very irregular, by which I don’t mean sometimes 26 days or sometimes 31 days apart, I’m talking months and months apart. The culprit of the hormonal disruption in PCOS is thought to be driven by insulin resistance, which affects the majority of women diagnosed with PCOS. Insulin is responsible for blood sugar control, and if your body develops a resistance to it, the pancreas responds by making more insulin. High insulin levels cause the ovaries to convert oestrogen into testosterone, which interferes with the signals for ovulation. And we now all know that if you don’t ovulate you don’t make a corpus luteum or progesterone to start the timer for your next period. You will still have plenty of oestrogen though, but it no longer has its friend progesterone to keep it in check. As a result, the endometrium can become too thick, so when bleeding does eventually happen, it’s often heavy with clots.
If you want to get pregnant, you need to be ovulating regularly, so PCOS can cause difficulties getting pregnant, although the majority of with PCOS are thought to get pregnant without requiring fertility treatment. However, if you are trying to get pregnant and have either known PCOS, or are having irregular periods, you should see your GP to discuss investigations and a possible referral to a fertility clinic after 6 months of trying. Unfortunately, many women who are diagnosed with PCOS report being dismissed and told to ‘come back when you want to have a baby’. This is a missed opportunity to provide essential health education about the implications of PCOS, which is associated with many other complications than fertility issues. PCOS is associated with a higher risk of high blood pressure, type 2 diabetes, gestational diabetes (which develops in pregnancy) and sleep apnoea. Lifestyle advice with regards to diet, exercise, sleep and stress management are crucial in PCOS, but actually for all of us, because as I’ve said a few times now, all the hormones talk to one another, so we need to keep our entire body in check to ensure that our ovaries are getting the right signals for a healthy menstrual cycle. It’s really important to lay a strong foundation for your gynae health, so the last four chapters of my first book ‘The Gynae Geek: your no-nonsense guide to down-there healthcare’ are dedicated to this.
- Soumpasis I, Grace B, Johnson S. Real-life insights on menstrual cycles and ovulation using big data. Hum Reprod Open. 2020;2020(2):hoaa011.
- Bull JR, Rowland SP, Scherwitzl EB, Scherwitzl R, Danielsson KG, Harper J. Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles. NPJ Digit Med. 2019;2:83.
- Practice Committees of the American Society for Reproductive Medicine and the Society for Reproductive Endocrinology and Infertility. Diagnosis and treatment of luteal phase deficiency: a committee opinion. Fertil Steril. 2021;115(6):1416-1423.
- Bozdag G, Mumusoglu S, Zengin D, Karabulut E, Yildiz BO. The prevalence and phenotypic features of polycystic ovary syndrome: a systematic review and meta-analysis. Hum Reprod. 2016;31(12):2841-2855.