Almost always oxidation: blood darkens as it ages - brown or rust at the start or end of a period, or in slow light months, is older blood that lingered before leaving. It looks alarming and means almost nothing alone.
The meaningful variations: bright red heavy flow matters only when soaking-heavy or clot-heavy; gray with a bad smell is infection (or a retained tampon - urgent), not period blood. The flags: missed period with dark spotting and one-sided pain (test - ectopic is same-day), recurring dark spotting between periods, or a change that persists across several cycles.
What to do
Common questions
Why is my period darker than usual?
Almost always: oxidation. Blood darkens as it ages and meets oxygen - brown or rust-colored blood is simply older blood that lingered in the uterus before leaving, which is why it appears at the start of a period (last month's tail end), at the end when flow slows, or during light months when everything moves slowly. It looks alarming and means almost nothing by itself. The variations with meanings: pink or watery can pair with very light flow or low estrogen; bright red and heavy is fresh, fast flow - concerning only when heavy (the soaking-a-pad-an-hour pattern) or clot-heavy; grayish discharge with a bad smell is not period blood at all - that is an infection sign (bacterial vaginosis or a retained tampon, which is urgent). Dark blood plus the flags that change the answer: a missed period with dark spotting and one-sided pain (rule out pregnancy including ectopic - test, and same-day care for pain), dark spotting between periods that keeps recurring (worth a polyps-or-hormones conversation), or any persistent change across several cycles. Pymander, a free AI doctor available 24/7 by text, can help you read your specific pattern.