Black period blood is almost always old blood that has taken extra time to leave your uterus. As blood sits in the uterine lining or moves slowly through the cervix, the iron in hemoglobin reacts with oxygen in a process called oxidation. This gradually shifts the color from red to brown to black. It looks alarming, but in most cases it’s completely harmless.
Why Blood Turns Black
Fresh blood is bright red because its hemoglobin is carrying oxygen. Once blood pools or slows down, that hemoglobin oxidizes, the same basic chemistry that turns a cut apple brown. The longer blood stays inside the body before exiting, the darker it gets. A slow trickle that takes hours or days to travel from the uterus through the cervix and out of the vagina will arrive looking dark brown or black, even though it started as ordinary red blood.
When Black Blood Is Normal
The most common time to see black or very dark brown blood is at the very beginning or very end of your period. At the start of a cycle, leftover blood from the previous period may have been sitting in the uterus for days or weeks. It finally gets pushed out as fresh bleeding begins, and by then it has fully oxidized. At the tail end of your period, the flow slows to a trickle, giving those last traces of blood plenty of time to darken before they exit.
Some people notice black blood consistently at these points in every cycle. Others see it only occasionally, depending on how heavy or light a particular period is, how active they’ve been, and how quickly their uterus sheds its lining. None of this signals a problem on its own.
Black Blood After Pregnancy
Postpartum bleeding (called lochia) follows a predictable color pattern. The first three to four days typically bring dark or bright red blood with small clots, flowing like a heavy period. Over the next week or so, the discharge shifts to pinkish brown and becomes thinner and more watery. By around day 12, it fades to a yellowish white and can continue as light spotting for up to six weeks, sometimes eight. Dark or black discharge early in this timeline is just older blood working its way out, but if it develops a foul smell or comes with a fever, that’s a sign of possible infection.
Implantation Bleeding
If you’re sexually active and notice dark spotting outside your expected period window, it could be implantation bleeding. This happens when a fertilized egg attaches to the uterine lining, roughly 6 to 12 days after conception. Implantation bleeding is typically brown, dark brown, or pink rather than the bright or dark red of a normal period. It’s also much lighter, usually just spotting that lasts a day or two. If the blood is very dark and arrives earlier or later than you’d expect your period, a pregnancy test can rule this in or out quickly.
Conditions That Can Cause Darker Blood
Endometriosis
Women with endometriosis sometimes report dark or old-looking blood appearing before their period starts or lingering after it ends. Endometriosis involves tissue similar to the uterine lining growing outside the uterus, where it still responds to hormonal cycles and bleeds. Endometrial tissue on the ovaries can form fluid-filled cysts sometimes called “chocolate cysts” because the trapped, oxidized blood inside them is dark brown. Dark blood alone doesn’t confirm endometriosis. The only definitive diagnosis requires a minor surgical procedure called a laparoscopy; scans and blood tests can come back normal even when the condition is present. Other symptoms to watch for include painful periods, pain during sex, and chronic pelvic pain.
Cervical Stenosis
In rare cases, the passageway through the cervix becomes unusually narrow or closes off entirely. When this happens, blood can’t flow out at a normal pace. It pools in the uterus, oxidizes, and comes out much darker than usual when it finally does pass through. A significant blockage can lead to blood accumulating in the uterus (a condition called hematometra), which sometimes causes pelvic pain or a feeling of pressure. Cervical stenosis can result from surgery, radiation, infection, or menopause-related changes.
Retained Tampon or Other Object
A forgotten tampon is more common than people think, and it can trap blood inside the vagina long enough for it to turn very dark. The telltale signs go beyond just color: a strong, foul smell is the most obvious red flag. Discharge may appear yellow, green, pink, grey, or brown. If you suspect a retained object, it can usually be removed easily during a quick medical visit.
Signs That Need Medical Attention
Black blood by itself, especially at the start or end of your period, rarely requires any action. But certain accompanying symptoms change the picture. Pelvic inflammatory disease, a bacterial infection of the reproductive organs, can produce unusual discharge with a bad odor, lower abdominal pain, fever, burning during urination, pain or bleeding during sex, and bleeding between periods. If your dark blood comes with any of those symptoms, it’s worth getting evaluated promptly.
The American College of Obstetricians and Gynecologists considers bleeding abnormal in several specific situations: soaking through a pad or tampon every hour for two or more hours in a row, periods lasting longer than seven days, cycles shorter than 21 days or longer than 35 days, spotting between periods or after sex, and missing your period for three to six months. If you’re soaking through products that fast and also feel dizzy, lightheaded, or short of breath, that warrants emergency care.
Outside of those scenarios, black or very dark blood is one of the most common and least concerning color variations in menstrual flow. Your body is simply showing you blood that took the slow route out.