Why Is My Period Blood Dark Brown or Black?

Period blood that comes out dark brown, almost black, or thick and tar-like is one of the most common menstrual changes women notice but rarely feel comfortable asking about. It is not something to ignore and it is not simply old blood in a way that means nothing. The colour of your menstrual blood tells you something specific about how well your uterus is clearing, how your hormones are behaving, and what your overall hormonal terrain looks like.

In functional medicine, menstrual blood characteristics are treated as diagnostic information. Colour, consistency, timing, flow volume, and associated symptoms all provide clues about the hormonal environment that produced that cycle. Dark brown or black period blood is one of the most consistently hormone-informative findings, and it almost always points toward a specific set of mechanisms worth understanding and addressing properly.

What period blood colour actually tells you

The colour of menstrual blood changes based on how long it takes to travel from the uterine lining through the cervix and out of the body. Fresh blood that exits quickly is typically bright red. Blood that sits in the uterus longer before being expelled oxidises, a process that darkens it progressively from red to brown to near-black. This oxidation is the same chemical process that turns a cut on your skin brown when it dries. The longer blood sits before being expelled, the darker it becomes.

A small amount of dark brown blood at the very start or end of a period is generally normal and simply reflects the slower movement of the first or last remnants of the uterine lining. The concern arises when the majority of a period is dark brown or black, when dark blood appears consistently across multiple cycles, when it comes with a thick or tar-like consistency, or when it is accompanied by painful cramps, heavy flow, or significant PMS.

Period blood colour is a window into your hormonal health. A predominantly dark brown or black period is your body’s way of communicating that the uterine environment is not clearing efficiently, and that the hormonal conditions producing this pattern deserve investigation.

The main causes of dark brown or black period blood

Oestrogen dominance: the most common hormonal cause

Oestrogen’s primary role in the first half of the menstrual cycle is to build the uterine lining in preparation for potential implantation. When oestrogen is elevated relative to progesterone, whether through excess production, impaired liver clearance, gut oestrogen recirculation, or insufficient progesterone to balance it, the uterine lining becomes thicker than normal. A thicker lining requires more time, more uterine contractions, and more prostaglandin activity to shed completely.

When the lining is not shed efficiently, portions of it remain in the uterus between periods and during the next menstrual flow, where they oxidise and turn brown or black before eventually being expelled. This is one of the primary mechanisms behind dark period blood and it is also associated with heavy periods, large clots, significant cramping, and the kind of painful periods that are frequently dismissed as normal but in functional medicine practice are understood as signs of a hormonally driven uterine environment accumulating more lining than it can cleanly shed.

Low progesterone and incomplete uterine shedding

Progesterone serves two critical functions in the context of healthy menstruation. During the luteal phase it stabilises and matures the uterine lining. When progesterone drops at the end of the cycle, it signals the uterus to begin the coordinated enzymatic and prostaglandin-driven process of complete lining shedding. When progesterone is insufficient, this shedding signal is weak and uncoordinated. The lining breaks down unevenly, shedding in fragments rather than as a complete, efficient process. Blood that exits during this fragmented shedding may have been pooling in areas of the uterus before the clearance signal was strong enough to expel it, resulting in significantly oxidised, dark-coloured flow.

Low progesterone is extremely common in South African women for the reasons covered in other articles in this series: chronic stress and the pregnenolone steal, anovulatory cycles, nutritional deficiencies in progesterone synthesis cofactors, and the post-pill hormonal recovery period. Women who notice their periods have become darker since coming off the pill, since a period of significant stress, or since entering their late thirties are frequently observing the effects of low progesterone on uterine shedding efficiency.

Dark period blood that appears throughout the period rather than just at the beginning and end is almost always a sign of either excess uterine lining from oestrogen dominance, incomplete shedding from low progesterone, or both operating simultaneously.

Endometriosis and adenomyosis: when dark blood signals something deeper

Endometriosis, where endometrial-like tissue grows outside the uterus, and adenomyosis, where it grows into the uterine muscle wall, are both associated with dark, thick, or black period blood. In endometriosis, the implants bleed during menstruation but have no exit route, causing blood to pool, oxidise, and eventually find its way into the menstrual flow over time. In adenomyosis, blood trapped within the uterine muscle wall produces a characteristic dark, heavy, and painful period associated with an enlarged or tender uterus.

Both conditions are significantly underdiagnosed in South Africa. The average time from symptom onset to endometriosis diagnosis is seven to ten years. Women with persistently dark, heavy, painful periods, particularly when accompanied by pelvic pain outside of menstruation, pain with intercourse, bladder or bowel symptoms around the period, or difficulty conceiving, should have endometriosis investigated properly rather than managed symptomatically with the contraceptive pill.

Thyroid dysfunction and uterine contractility

The uterus requires adequate thyroid hormone to contract efficiently during menstruation. Hypothyroidism impairs uterine smooth muscle contractility, reducing the strength and coordination of the contractions that expel the uterine lining. The result is a slower, less complete menstrual flow that gives blood more time to oxidise within the uterine cavity before exiting, producing darker menstrual blood and often a heavier, more prolonged period.

Thyroid dysfunction also contributes to dark periods through its effect on prolactin. Hypothyroidism stimulates TRH production, which cross-stimulates prolactin secretion. Elevated prolactin suppresses ovulation and progesterone production, creating the oestrogen-dominant, low-progesterone hormonal environment that directly produces dark, sluggish menstrual flow.

Iron deficiency and blood viscosity

Iron deficiency, particularly low ferritin, affects menstrual blood through its impact on blood viscosity and uterine tissue quality. Iron-deficient blood is less fluid, and uterine tissue that is nutritionally compromised does not shed with the same efficiency as well-nourished tissue. Women with low ferritin frequently report that their period blood has become thicker, darker, and more clotted over time, which worsens the iron loss with each cycle and creates a compounding cycle of deficiency and poor menstrual flow quality.

Hemagenics provides comprehensive iron and haematinic support formulated for absorption and tolerability, addressing the iron deficiency component that contributes to dark, thick menstrual flow in many South African women.

When dark period blood requires urgent investigation

While most cases of dark period blood reflect the hormonal mechanisms described above and are addressable with a functional medicine approach, certain presentations require prompt medical investigation. You should see a practitioner promptly if dark blood is accompanied by severe pelvic pain that is worsening over time, if periods have become significantly heavier alongside the colour change, if dark spotting occurs between periods or after intercourse, if there is fever or an unusual odour associated with the menstrual blood, or if you are trying to conceive without success alongside these menstrual changes.

A functional medicine approach to dark period blood

Addressing dark period blood from a root cause perspective requires identifying which hormonal mechanism is driving the slow uterine clearance. A thorough investigation includes a full hormone panel timed correctly to the cycle, including oestradiol, progesterone at day 21, testosterone, DHEA-S, and SHBG, alongside a full thyroid panel, iron studies including ferritin, inflammatory markers, and where symptoms suggest endometriosis, appropriate imaging and specialist referral.

The nutritional protocol targets the specific drivers identified. For oestrogen dominance, supporting liver oestrogen clearance through DIM and calcium d-glucarate is foundational. EstroFactors combines both in a formulation designed specifically for healthy oestrogen metabolism, while Meta I 3 C provides additional Phase 1 liver detoxification support. For low progesterone, Chasteberry Plus supports luteal phase progesterone production. OmegaGenics EPA-DHA 1000 reduces the prostaglandin-driven inflammation that impairs efficient uterine shedding.

Targeted support for dark period blood and menstrual health

The bottom line

Dark brown or black period blood is not simply old blood that means nothing. It is a consistent clinical signal that uterine clearance is slow, that the uterine lining has accumulated beyond what the body can shed efficiently, or that blood is pooling and oxidising before exiting. In the vast majority of cases this points toward oestrogen dominance, low progesterone, thyroid dysfunction, iron deficiency, or a combination of these, all of which are identifiable through proper investigation and addressable with targeted hormonal and nutritional support.

Understanding your hormonal pattern is the essential first step. The free hormone assessment quiz at Hormone Reset helps identify which imbalances are most likely behind your menstrual blood changes, so you can move from observation to investigation to targeted treatment rather than managing symptoms indefinitely.

Your period blood is diagnostic. Its colour, consistency, and timing are your body’s most direct monthly communication about the hormonal environment inside your uterus. Dark blood is not something to normalise. It is something to investigate.