Originally written by ZRT Laboratory author. Reproduced with permission. Last reviewed: May 2026.
Menstruation is a fundamental aspect of female reproductive health, yet heavy or painful periods remain one of the most commonly dismissed symptoms in women's healthcare. Understanding what drives your menstrual cycle — and what can disrupt it — is the first step towards taking meaningful action.
This article explains the physiology of the menstrual cycle, the most common causes of heavy bleeding, and how hormone testing can help identify what is really going on.
What Actually Happens During a Period?
A period — or menstruation — is the shedding of the uterine lining (endometrium) that occurs when a fertilised embryo is not implanted. The endometrium is made up of mucosal tissue with two functional layers attached to the smooth muscle wall of the uterus. Oestrogen stimulates its growth during the first half of the cycle; progesterone helps it mature and stabilise during the second half.
Humans are among a small group of mammals — alongside apes, monkeys, bats, and elephant shrews — that shed this lining each cycle rather than reabsorbing it. When pregnancy does occur, the endometrium becomes the nourishing environment in which the embryo implants and begins to develop.
Average menstrual blood loss is approximately 40 ml per cycle (around two to three tablespoons), though some women lose considerably more. It is entirely normal to use between one and seven standard-sized tampons or pads per period. Because even small volumes of blood can appear visually alarming — particularly in water — the amount lost can feel far greater than it actually is.
One important point worth clarifying: women have no voluntary control over when or how long they bleed. The uterus does not have a sphincter muscle, and the cervix cannot be used to delay or pause menstruation.
What Causes Heavy Periods?
Heavy menstrual bleeding — clinically referred to as menorrhagia — affects approximately one in three women at some point in their lives. If you are saturating a pad or tampon every hour for several consecutive hours, it is important to speak with your GP. There are several well-established causes worth understanding.
Hormonal Imbalance: Too Much Oestrogen, Too Little Progesterone
One of the most common drivers of heavy periods is an imbalance between oestrogen and progesterone. When oestrogen levels are relatively high and progesterone is insufficient — a pattern sometimes referred to as oestrogen dominance — the endometrium continues to grow without adequate maturation or stabilisation. The result is a thicker, more vascular lining that sheds more heavily.
This pattern is particularly common in the years approaching perimenopause, when ovulation becomes less consistent and progesterone production declines. It can also occur in younger women with conditions such as polycystic ovary syndrome (PCOS) or luteal phase defects, where progesterone output after ovulation is insufficient.
Hormone testing across the full menstrual cycle can reveal whether this pattern is present. Our 1 Month Hormone Test for Women maps oestrogen and progesterone across all 28 days of your cycle using daily dried urine samples, providing a detailed picture of whether ovulation is occurring and whether progesterone is rising adequately in the luteal phase.
Uterine Fibroids
Fibroids are benign growths of smooth muscle tissue within or around the uterine wall. They are extremely common — affecting up to 70% of women by the age of 50 — and are one of the leading causes of heavy menstrual bleeding and hysterectomy. The location and size of fibroids significantly influence their impact on bleeding: submucosal fibroids (those that protrude into the uterine cavity) tend to cause the heaviest bleeding.
Fibroids are oestrogen-sensitive, meaning they tend to grow in response to higher oestrogen levels and often shrink after the menopause. Identifying and addressing hormonal drivers alongside structural management is therefore an important part of a comprehensive approach.
Adenomyosis
Adenomyosis occurs when endometrial tissue grows into the muscular wall of the uterus itself. Because the uterine muscle has limited ability to contract around this displaced tissue, bleeding can be heavy, prolonged, and painful. The uterine lining may also be thicker than usual. Adenomyosis is more common in women who have had multiple pregnancies, a history of uterine procedures (such as caesarean sections or myomectomies), or an invasive placenta during childbirth.
Structural Abnormalities
Variations in uterine anatomy — including a bicornuate (heart-shaped) uterus, uterine septum, or the presence of polyps — can interfere with the uterus's ability to contract efficiently and stop bleeding. Uterine polyps, which are small, benign growths of endometrial tissue, are a particularly common and often overlooked cause of heavy or irregular bleeding.
Bleeding Disorders
For some women, particularly adolescents experiencing their first heavy periods, an underlying bleeding disorder may be responsible. Von Willebrand disease — the most common inherited bleeding disorder — can cause significantly heavier-than-normal menstrual flow and is frequently underdiagnosed in women. If heavy periods have been present since menarche, it is worth discussing this possibility with your GP.
Iron Deficiency and Anaemia
Iron deficiency and heavy periods create a self-reinforcing cycle. Heavy bleeding depletes iron stores, and iron deficiency in turn impairs the contractility of uterine muscle — reducing the uterus's ability to clamp down and limit blood loss. Women with heavy periods should have their iron and ferritin levels checked regularly, as treating iron deficiency can itself help reduce menstrual blood loss over time.
How Hormones Drive the Menstrual Cycle
Understanding the hormonal architecture of the menstrual cycle helps explain why imbalances have such a direct impact on bleeding patterns.
The Follicular Phase (Days 1–14)
The cycle begins on the first day of menstruation. Rising levels of follicle-stimulating hormone (FSH) stimulate the development of ovarian follicles, which produce oestrogen. Oestrogen drives the proliferation of the endometrium, preparing it to receive a fertilised egg. As oestrogen peaks, a surge in luteinising hormone (LH) triggers ovulation — the release of a mature egg from the dominant follicle.
The Luteal Phase (Days 15–28)
After ovulation, the ruptured follicle transforms into the corpus luteum, which produces progesterone. Progesterone is responsible for maturing and stabilising the endometrium, suppressing further growth, and preparing the lining for potential implantation. If fertilisation does not occur, the corpus luteum degenerates, progesterone falls, and menstruation begins.
When progesterone production in the luteal phase is insufficient — whether due to poor ovulation, a short luteal phase, or corpus luteum dysfunction — the endometrium does not mature properly. This can result in heavier, more prolonged, or irregular bleeding.
If you suspect your hormones may be contributing to heavy periods, our Advanced Female Wellness Test provides a comprehensive assessment of oestrogen, progesterone, testosterone, DHEA, cortisol, and thyroid markers — with a specialist doctor's report included.
When Should You Seek Medical Advice?
You should speak with your GP if you experience any of the following:
- Soaking through a pad or tampon every hour for two or more consecutive hours
- Passing blood clots larger than a 50p coin
- Periods lasting longer than seven days
- Bleeding between periods or after sex
- Significant pelvic pain or pressure
- Symptoms of anaemia, including fatigue, breathlessness, or dizziness
Heavy periods are not something you simply have to endure. Effective treatments exist — from hormonal management and iron supplementation to surgical options for structural causes — and the right approach depends on identifying the underlying driver.
How Hormone Testing Can Help
For many women, the root cause of heavy periods is hormonal — but standard GP blood tests taken on a single day often fail to capture the full picture. Oestrogen and progesterone fluctuate significantly across the cycle, and a snapshot taken at the wrong time can appear entirely normal even when an imbalance is present.
Comprehensive hormone testing across the full cycle — or at minimum during the mid-luteal phase when progesterone should be at its peak — provides far more clinically meaningful data. At-home dried urine testing makes this accessible without repeated clinic visits.
Our Elite Hormone Test measures a broad panel of reproductive hormones, adrenal markers, and metabolites using dried urine — the same methodology used by specialist functional medicine practitioners — giving you and your healthcare provider the detail needed to understand what is driving your symptoms.
Frequently Asked Questions
What is considered a heavy period?
Clinically, heavy menstrual bleeding (menorrhagia) is defined as blood loss exceeding 80 ml per cycle. In practical terms, this means soaking through a pad or tampon every hour for several hours, passing large clots, or having periods that last longer than seven days. However, any bleeding that significantly disrupts your daily life warrants investigation, regardless of volume.
Can hormone imbalance cause heavy periods?
Yes. An imbalance between oestrogen and progesterone — particularly when oestrogen is relatively high and progesterone is insufficient — is one of the most common hormonal causes of heavy periods. This pattern causes the endometrium to grow excessively without adequate stabilisation, resulting in heavier shedding. Hormone testing across the full menstrual cycle can identify this pattern.
What is the difference between fibroids and adenomyosis?
Fibroids are discrete benign tumours of smooth muscle that grow within or around the uterine wall. Adenomyosis involves endometrial tissue growing into the uterine muscle itself, without forming a distinct mass. Both can cause heavy, painful periods, but they are different conditions requiring different management approaches. Both can coexist in the same individual.
Can stress affect my period?
Yes. Chronic stress elevates cortisol, which can suppress the hormonal signals needed for regular ovulation. Without consistent ovulation, progesterone production in the luteal phase is reduced, which can lead to heavier, irregular, or missed periods. Addressing adrenal and cortisol health is therefore an important part of managing menstrual irregularity.
How can I tell if my progesterone is low?
Symptoms of low progesterone include heavy or prolonged periods, spotting before your period, a short luteal phase, PMS, anxiety, poor sleep, and difficulty conceiving. The most reliable way to confirm low progesterone is through hormone testing during the mid-luteal phase (approximately seven days after ovulation), when progesterone should be at its highest point in the cycle.
Is it normal to pass clots during a period?
Small clots (smaller than a 50p coin) are common and generally not a cause for concern. Larger or more frequent clots may indicate heavier-than-normal bleeding and can be associated with hormonal imbalance, fibroids, or adenomyosis. If you are regularly passing large clots, it is worth discussing this with your GP.
Originally written by ZRT Laboratory author. Reproduced with permission. Last reviewed: May 2026.