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what does menstrual blood look like

If it’s pinkish…

You may have low estrogen levels. Especially if it’s accompanied by a lighter-than-usual flow, or if you’re an avid runner, says New York-based functional medicine nurse practitioner Margaret Romero. Studies have found that excessive exercise can lower estrogen levels, which can subsequently mess with your period, sometimes causing it to disappear altogether. (It’s not uncommon for female professional athletes to stop ovulating.)

While this may not seem like a big deal (who hasn’t fantasized about never having to deal with a period at least once or twice?), low estrogen levels can increase your risk of osteoporosis if left untreated. So if you’ve recently started training for a marathon, have started working out for the first time in your life, or have upped the intensity of your workouts and you notice that your periods are suddenly lighter in color and flow or less frequent, talk to your doctor.

If it’s watery-looking…

You may have a nutritional deficiency. Ob-gyn Alyssa Dweck, MD, assistant clinical professor at the Mount Sinai School of Medicine in New York, says that a white-ish, diluted flow can be a symptom of severe anemia, especially if you notice your period getting lighter and lighter when it would ordinarily get a bit heavier. If, after monitoring your period for two or three cycles, you’re worried that this could be the case, talk to your doctor about getting tested for nutritional deficiencies.

To make matters a little more confusing, an iron deficiency might instead be caused by heavy periods. (A 2014 Finnish study that examined 236 women with heavy periods found that 27% were iron deficient and 60% were severely anemic.) If you bleed through pads or tampons in less than an hour, wake up at night to change your pads or tampons several times, or regularly feel tired and foggy, it can’t hurt to get your iron level checked.

If it’s dark brown…

You may have older bits of uterine lining and blood that are just now making their way out of your body. But don’t panic: This is normal. “We’re not sure why this happens in all cases, but sometimes the blood is sitting around for a while and comes out particularly slowly,” Dweck says. “It has a lot of time to oxidize, which is why it can look brown or almost black.” (Just make sure you see your ob-gyn if you experience any of these period symptoms.)

Everyone will shed her uterine lining at a different rate (like snowflakes, each period is unique), but for the most part, seeing some dark brown blood at the beginning of your period or toward the end of it is nothing to worry about.