Black period blood 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, gradually shifting from bright red to dark red, then brown, and finally black. This is the same reason a cut on your skin turns dark as it scabs over. It looks alarming, but in most cases it’s completely normal.
Why Blood Turns Black
Fresh blood is bright red because of the iron-rich protein in your red blood cells that carries oxygen. Once blood is exposed to air and starts to break down, that protein changes shape and darkens. The longer blood stays inside the body before it exits, the more time it has to oxidize. Blood that has been sitting for hours or days can turn so dark it looks black.
This process happens at a pace that matches your flow. On heavy days, blood moves through quickly and tends to look bright or dark red. On lighter days, blood trickles out slowly and has much more contact time with oxygen before you see it. That’s why black or very dark brown blood shows up most often at the very beginning or very end of a period, when your flow is at its lightest.
Common, Harmless Causes
Start or End of Your Period
The most frequent explanation is simply timing. The first day or two of a period can release blood that’s been lingering in the uterus since your last cycle ended. The final days of a period produce only small amounts of blood that move slowly. In both situations, the result is dark brown to black spotting that’s entirely routine.
Irregular or Light Periods
If your cycles are irregular or your flow is naturally light, blood may spend extra time in the uterus before being shed. Hormonal shifts from stress, weight changes, or breastfeeding can slow things down enough to produce darker blood. This doesn’t signal a problem on its own.
Postpartum Bleeding
After giving birth, the body sheds blood and uterine tissue over a period of about six weeks. This discharge, called lochia, starts as dark or bright red bleeding in the first three to four days, then becomes pinkish-brown for roughly a week, and eventually turns a yellowish white. Some of the early discharge can appear very dark, especially if blood pools while you’re lying down and then passes when you stand up. The overall pattern should be one of gradually lightening color and decreasing volume.
Less Common Causes Worth Knowing
Retained Object in the Vagina
A forgotten tampon, a piece of a broken condom, or a dislodged contraceptive device can trap blood and tissue, giving it time to oxidize to a very dark color. The key giveaway is usually a strong, foul smell alongside discolored discharge that may be brown, gray, or black. Other signs include pelvic pain, swelling, itching, fever, or discomfort when urinating. If this combination sounds familiar, a healthcare provider can safely remove the object, and symptoms typically resolve quickly.
Cervical or Vaginal Blockage
Rarely, a narrowing of the cervical opening (cervical stenosis) can partially or fully block menstrual blood from exiting. This can happen after cervical surgery, radiation treatment, or certain procedures on the uterus. It can also be congenital, caused by an imperforate hymen or a wall of tissue across the vaginal canal. When blood is trapped and accumulates, the uterus becomes distended in a condition called hematometra. If the blockage is incomplete, you may notice dark brownish-black spotting. If it’s complete, periods may stop altogether while cramping continues in a cyclical pattern. Doctors diagnose it with an ultrasound or by attempting to pass a thin probe through the cervix, which releases the accumulated dark blood.
Miscarriage
In early pregnancy, dark brown or black spotting can sometimes indicate a missed miscarriage, where the pregnancy has stopped developing but the tissue hasn’t been expelled. UC Davis Health describes this discharge as looking like coffee grounds: old blood leaving the uterus very slowly. There’s typically no heavy bleeding, which is part of why missed miscarriages can go unrecognized until an ultrasound reveals the loss. If you know or suspect you’re pregnant and see persistent dark discharge, that’s worth having evaluated.
Infection
Pelvic inflammatory disease and other reproductive tract infections don’t typically cause black discharge on their own, but an infection combined with slow-moving blood can produce unusually dark, foul-smelling discharge. The CDC lists pain in the lower abdomen, fever, a bad-smelling vaginal discharge, pain during sex, and bleeding between periods as signs of PID. If old blood trapped in the uterus becomes infected, the odor becomes distinctly unpleasant. Smell is the distinguishing factor here: normal oxidized blood doesn’t have much of an odor beyond a faint metallic quality.
Endometriosis and PCOS
There’s no strong evidence that conditions like endometriosis or polycystic ovary syndrome change the color of period blood in a unique way. People with endometriosis have the same range of blood colors as anyone else. That said, advanced endometriosis can produce ovarian cysts filled with old blood, sometimes called chocolate cysts. If one of these cysts ruptures, it can leak dark brown fluid into the pelvic cavity, which may show up as very dark discharge. PCOS often causes irregular or infrequent periods, which means blood can sit longer in the uterus between cycles and appear darker when it finally arrives.
Signs That Need Attention
Black blood by itself, appearing for a day or two at the start or end of your period, is normal. But certain combinations of symptoms suggest something beyond simple oxidation:
- Foul smell: A strong, unpleasant odor alongside dark discharge points toward infection or a retained object.
- Fever: Any fever paired with unusual discharge warrants prompt evaluation.
- Persistent black discharge outside your period: Dark spotting that continues for days between cycles or appears when you’re not expecting a period is worth investigating.
- Severe or worsening pelvic pain: Cyclical cramping without a visible period can signal a blockage. Sharp or escalating pain could indicate a ruptured cyst or ectopic pregnancy.
- Possible pregnancy: Dark discharge during early pregnancy, especially with cramping, could indicate miscarriage.
- Soaking through a pad or tampon every hour: Extremely heavy bleeding of any color, particularly with large clots, suggests something beyond routine menstruation.
If your black period blood shows up predictably at the tail end of your cycle, lasts a day or so, and doesn’t smell unusual, it’s simply your body clearing out the last of the lining at its own pace. The color reflects time, not danger.