How does blood color vary during a period?
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.