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Why Is My Period Blood Brown? Dark Shades of Blood During Menstruation

Periods are different for everyone, but a lot of people have something in common: the color of their period blood can change over the course of a period. Sometimes it’s bright red, while other times it looks brown or black. Having brown blood on your period is usually normal, but why does it happen?

The color of your period blood can vary throughout your period, and that’s normal. When blood comes into contact with air, it goes through a natural process called oxidation. Hemoglobin is a blood compound that transports oxygen and contains iron. When this iron is exposed to air and oxygen, it becomes iron oxide (the same compound as rust), which has a darker brown color. This explains why your period blood is dark brown sometimes, and it can even look black in some cases.

You’re more likely to see this brown period blood on the first day and last days of your period, when the flow is lighter, but it’s also normal for it to be bright red at the beginning. Blood tends to be a brighter shade of red during the middle of your period, since the flow is faster, giving the blood less time to oxidize before leaving your uterus. However, you could still see some dark blood clots during heavier flow days.

Seeing a range of red and brown shades during your period is perfectly normal, and unless you experience other troubling period-related symptoms, you probably don’t have anything to worry about.

Here are some warning signs that you may need to visit your doctor:

  • Abnormal vaginal discharge (white, yellow, green, grey, foul-smelling, thick, or frothy)
  • Intense lower abdominal pain during your period or otherwise
  • Periods that are too short (less than 2 days) or too long (more than 7 days)
  • Periods that are too heavy (having to change period products every 2 hours or less)
  • Periods that are too close together (less than 21 days) or too far apart (more than 35 days)
  • Bleeding or spotting outside of your menstrual cycle or after intercourse
  • Severe PMS symptoms (nausea, vomiting, headaches, bloating, and mood swings, among others)
  • Very irregular and unpredictable cycles

Now you know why period blood is darker sometimes, but why is your period blood brown at the beginning? For lots of folks, the first and last days of their period are usually the lightest, and some people experience spotting for a few days after the end of their period.

Because blood flow during these days is slower and lighter, it takes longer to leave your uterus. This gives the blood plenty of time to oxidize and become brown, instead of the brighter red that is more frequent during the heaviest days of menstruation.

Seeing different shades of red, brown, and even black blood during your period is typically normal and nothing to worry about. Blood contains hemoglobin and iron, a mineral that oxidizes when it comes into contact with air. When this occurs, it causes blood to become darker.

Dark brown period blood is most common at the beginning and end of periods. During these days, blood flow tends to be slower, which gives the blood more time to oxidize before it leaves the uterus.

However, it’s always a good idea to bring up any concerns about your reproductive health with your health care provider and to schedule regular checkups. Your physician will be able to address any concerns you may have about your periods and sexual health.

“Why Dose Blood Turn Red When It Touches the Air?” UCSB Science Line, 2006, scienceline.ucsb.edu/getkey.php?key=1329.

“Heavy Menstrual Bleeding.” Centers for Disease Control and Prevention, 20 Dec. 2017, www.cdc.gov/ncbddd/blooddisorders/women/menorrhagia.html.

“Menstrual Cycle: What's Normal, What's Not.” Mayo Clinic, Mayo Foundation for Medical Education and Research, 29 Apr. 2021, www.mayoclinic.org/healthy-lifestyle/womens-health/in-depth/menstrual-cycle/art-20047186?pg=1.

“Abnormal Menstruation (Periods): Types, Causes & Treatment.” Cleveland Clinic, 25 Aug. 2019, my.clevelandclinic.org/health/diseases/14633-abnormal-menstruation-periods.

Dasharathy, Sonya S et al. “Menstrual bleeding patterns among regularly menstruating women.” American journal of epidemiology vol. 175,6 (2012): 536-45. doi:10.1093/aje/kwr356

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