Why is my period blood black?
Black period blood is usually old blood. The longer blood stays in the uterus or vagina, the more it reacts with oxygen and darkens from red to brown to almost black. That's why it often shows up at the very start or end of a period, when flow is slow. It's normally harmless unless it comes with a bad smell, fever or pain.
Why blood turns black
Fresh blood is bright red. Once it's exposed to air it oxidizes, the same way a cut apple browns. On slow days, blood can sit in the uterus or vagina for a while before it leaves, so by the time you see it, it's dark brown or black. Nothing is wrong with the blood itself; it's just older.
| When you see it | What it usually means |
|---|---|
| First day or two, before flow picks up | Leftover blood from last cycle, or slow early flow |
| Last day or two | Flow is slowing and the last blood is taking longer to come out |
| Mixed with clots on heavy days | Blood that pooled before it left; see are clots normal |
| Spotting between periods | Old blood from light bleeding; worth tracking if it keeps happening |
For the full color guide (bright red, pink, brown, orange, gray) see period blood colors. Brown has its own page too: why is period blood brown.
When black blood is a sign of something else
Dark blood alone is rarely a problem. It's the company it keeps that matters:
- A strong, rotten smell, with dark discharge: this can mean a forgotten tampon or other object in the vagina. It's common, nothing to be embarrassed about, and a doctor or nurse can remove it quickly.
- Gray, green or fishy-smelling discharge: possible bacterial vaginosis or an STI.
- Dark bleeding with cramping in early pregnancy: possible miscarriage. Take a test and contact a doctor.
- Dark spotting after a missed period: take a pregnancy test; very rarely, it can be a sign of ectopic pregnancy if there's one-sided pain.
- Dark bleeding after sex or after menopause: always worth getting checked.
See a doctor quickly if black or dark bleeding comes with a fever, a foul smell, itching, severe or one-sided pain, dizziness, or if you could be pregnant. Any bleeding after menopause (12 months with no period) needs a check, even if it's light.
What helps you tell normal from not
Patterns are more useful than single days. If dark blood shows up on day 1 or the last day of most periods, that's just how your body works. If it's new, keeps happening between periods, or arrives with other symptoms, that's worth noting. Logging flow and color for a couple of cycles makes it easy to describe to a doctor, and it often shows you there's nothing to worry about.
Periods can feel a bit unsettling when something looks different, and it's easy to spiral at 2am. Remember that your flow naturally starts slow, peaks around days 2 and 3, and slows again. Dark blood at the edges of that curve is expected.
Frequently asked questions
Is black period blood a sign of infection?
Not on its own. It becomes a concern when it comes with a bad smell, fever, itching or unusual discharge.
Why is my period black and thick?
Thicker, darker blood is usually old blood mixed with uterine lining, often at the start or end of your period. Thick clots larger than a quarter, often, are worth mentioning to a doctor.
Can stress make period blood darker?
Stress can make flow lighter or slower, which gives blood more time to darken. It doesn't change the blood itself.
Is black blood at the start of a period normal?
Yes. It's often leftover blood from your last period that didn't fully clear, coming out as flow begins.
Sources
- NHS: Periods
- NHS: Vaginal discharge
- ACOG: Abnormal Uterine Bleeding FAQ
- Cleveland Clinic: Menstrual Cycle
This page is for information only and isn鈥檛 medical advice. If something worries you, please talk to a doctor.
Keep reading
What color should period blood be? A simple color guidePeriod blood color guide: bright red, dark red, brown, black, pink, orange and gray, what each usually means, and which colors are worth a doctor's check.Why is my period blood brown?Brown period blood is usually just older blood that took longer to leave your body. Why it shows up at the start or end of a period, and when to check it.Are blood clots normal during your period?Are blood clots normal during your period? Small ones usually are. Why clots form, what size is too big, and when clots are a sign to see a doctor.
Brown discharge: what it usually means and when to checkBrown discharge is usually old blood. What it means before, after or instead of your period, in early pregnancy and perimenopause, and when to see a doctor.
Period calculator: when is my next period?Free period calculator: enter the first day of your last period and your cycle length to see your next 6 periods. Private, instant, nothing is stored.Why does period blood smell?Why does period blood smell? A light metallic smell is normal. What different period smells mean, how to stay fresh, and when an odor needs a doctor.