Just because your period happens every month doesn't mean you always know what to expect. There's the cramps that come out of nowhere, the sometimes unpredictable start dates — and then there's the color of your actual period blood, which can sometimes throw you for a bit of a loop. I mean, sure, it's red, but is it the right red? "Women tend to worry when their period isn't exactly the same as usual,"says Raquel B. Dardik, MD , an ob/gyn at NYU Langone Medical Center, "but there's a large variety of what's normal." One thing that's almost always normal: Noticing that the color of your blood changes throughout the course of your period. That's because the most significant factor in the color of your period blood is the sheer quantity of it, Dr. Dardik explains. "If you just have a little bit of bleeding, it doesn’t come out right away," she says. That means it'll spend more time getting oxidized, which gives it a brownish hue. If you're on hormonal birth control, you may see lighter red or brown blood more often because your period is (in general) going to be lighter than those who aren't taking it. The consistency of your blood — specifically, the presence of any clots — may vary as well. When your uterine lining sheds, your body produces anticoagulant compounds that help break it up into an easily "passable" liquid form. But if your flow is moving quickly and heavily, those compounds don't always have enough time to work and you end up with a few dark, purplish-red jelly-like clots. Dime- or quarter-sized clots are totally normal, says Dr. Dardik, but anything bigger than that suggests you're losing a lot of blood very quickly, which is definitely not a good thing and means you should give your doctor a call. But if you're not on hormonal birth control and you don't have other issues related to your period (such as PCOS, which can cause an unpredictable flow), here's what your menstrual blood is telling you: