This question is from a reader. Feel free to contact me with questions!
My menstrual blood is sometimes brown instead of red. Is there something wrong with me?
No, you're fine.
Blood is bright red when it is oxygenated. Blood has hemoglobin, which can bind oxygen molecules. In the lungs, oxygen is passed from the lungs into the blood, turning it bright red. As the blood is transported through the body, its oxygen is transferred to the organs it passes through, and the blood becomes darker (dark red, bluish). By the time it reaches the veins (which bring blood back to the heart), much of the oxygen content has reduced.
During menstruation, blood is shed by the lining of the uterus, and that is usually bright red blood. The blood does not always come out immediately. It can sit in the uterus, cervix or vagina for some time before the woman sees the blood. In that time, the blood can be oxidized (which is different from oxygenated) - it involves a transfer of electrons, rather than just binding of oxygen to hemoglobin. When blood is oxidized, it turns brownish or black.
Therefore, brownish period blood is merely blood that has been sitting around for a while. All blood, after being exposed to air, will eventually turn brown or black.
(Incidentally, this is why stool can sometimes be black - because there is gastrointestinal bleeding. If you have black stools, see your doctor)