Black blood during your period is almost always old blood that has taken longer than usual to leave your uterus. As blood sits in the uterus or vaginal canal, it reacts with oxygen in a process called oxidation, shifting from red to brown to dark red and eventually to black. This is the same reason a cut on your skin turns dark as it heals. It’s extremely common and, on its own, not a sign of anything wrong.
Why Blood Turns Black
Fresh menstrual blood is bright or dark red because it’s moving quickly. When flow slows down, blood pools in the uterus or takes a longer path out, giving it more time to oxidize. The longer blood is exposed to air, the darker it gets. Think of it like leaving a drop of blood on a tissue: it starts red and gradually turns nearly black over a few hours. The same chemistry happens inside your body.
This is why black or very dark brown blood shows up most often at the very beginning or very end of your period. At the start, you may be shedding small amounts of blood left over from the previous cycle that stayed in the uterus. At the end, flow slows to a trickle, and the remaining blood oxidizes before it exits. Both situations are completely normal.
Start and End of Your Period
Most people notice black or dark brown spotting for a day or two before their period fully picks up, then again during the final day or two as it tapers off. During the middle of your period, when flow is heaviest, blood exits faster and tends to look bright or dark red. The pattern of dark-light-dark across a single cycle is one of the most common menstrual experiences, and it reflects nothing more than the speed of flow changing over those days.
If you use a menstrual cup or disc, you might notice the blood at the bottom of the cup looks darker than what’s on top, simply because it collected first and sat longer. Similarly, blood on a pad that’s been worn for several hours will look darker than what you see on a freshly changed one. The collection method and how long blood sits before you see it both affect color.
Implantation Bleeding vs. a Period
If you’re trying to conceive or think you might be pregnant, dark spotting between periods can raise questions about implantation bleeding. Implantation bleeding happens when a fertilized egg attaches to the uterine lining, typically around 10 to 14 days after conception. It’s usually pink or brown, very light in flow (more like discharge than a period), and shouldn’t soak through a pad or produce clots.
If the blood you’re seeing is dark red, heavy, or contains clots, it’s more likely a regular period than implantation bleeding. The key differences are volume and duration: implantation bleeding is brief (a day or two of spotting) and light enough that a thin liner is all you need.
Postpartum Bleeding
After giving birth, you’ll experience lochia, a discharge that follows a predictable color timeline. For the first three to four days, lochia is dark or bright red, heavy, and may include small clots. Over the next week or so, it transitions to a pinkish brown. By about 10 to 14 days postpartum, it shifts to a creamy, yellowish white that can last up to six weeks. Seeing very dark or nearly black blood in the first few days after delivery fits this normal pattern. If heavy, bright red bleeding returns after it had already started tapering, that’s worth a call to your provider.
When Black Blood Can Signal a Problem
On its own, black period blood is not concerning. But when it shows up alongside other symptoms, it can point to something that needs attention.
Retained Object
A forgotten tampon or other object in the vagina can cause blood to pool and darken. The hallmark sign is a strong, foul smell from the vaginal area, often accompanied by unusual discharge that may be yellow, green, gray, or brown. If you notice a bad odor along with dark discharge, check whether a tampon or menstrual product may still be in place.
Infection
Pelvic inflammatory disease and other infections can cause unusual vaginal discharge, sometimes with a foul smell. Other signs include lower belly or pelvic pain, pain during sex, fever or chills, bleeding between periods, and a burning sensation when you urinate. PID sometimes causes only mild symptoms or none at all, so persistent pelvic pain or discharge that smells off warrants attention even if the symptoms feel minor.
Cervical Stenosis
In rare cases, the cervical opening can become abnormally narrow or even close off, which slows or blocks menstrual flow. When blood can’t exit at a normal pace, it may oxidize heavily and appear very dark. Signs include increasingly painful periods, lighter or absent flow despite cramping, and in some cases, blood flowing backward into the pelvis. This is uncommon but worth knowing about if you’ve noticed your periods becoming progressively more painful while flow seems to decrease.
Pregnancy Loss
Early miscarriage can involve bleeding that ranges from light brown spotting to heavy, period-like flow. A missed miscarriage, where the pregnancy has ended but the body hasn’t yet responded, may produce no symptoms at all initially, then dark spotting or discharge. Light bleeding before 12 weeks of pregnancy is common and often harmless, but any bleeding during pregnancy is worth reporting to your provider regardless of color.
Red Flags to Watch For
The color of your period blood alone rarely indicates a medical problem. What matters more is the company it keeps. Pay attention if dark or black discharge comes with any of the following:
- A foul or fishy odor that’s different from the normal metallic smell of menstrual blood
- Discharge that looks like cottage cheese or pus, or is green, yellow, or gray
- Itching, burning, or swelling around the vagina
- Pelvic pain or cramping that’s significantly worse than your usual period
- Fever or chills
Without those accompanying symptoms, black blood at the start or end of your period is simply old blood making its way out on its own schedule. It’s one of the most common color variations in menstrual flow and, for most people, completely routine.