Have you ever wondered why the colour of period blood changes through the menstrual cycle? The colour of period blood can tell us a lot about what’s going on with our body.
Multiple factors affect the colour spectrum of our menstrual blood. It’s important to note that variants in colour are totally normal; you may see bright red blood one day, and rusty brown the next.
So, let’s talk about why our period may change in colour.
Our period blood is a mixture of endometrial cells (the tissue lining the inner uterus), blood, and vaginal secretions1. The colour of blood we see is a result of the speed of period flow and exposure to oxygen.
Flow speed + oxygen
Think about this: when you nick yourself shaving, you will see bright red blood form instantly, but the next day it will be a darker red scab. The same concept occurs with menstruation. If the uterus is contracting blood out quickly, it’s likely to be a brighter red. Whereas, when the uterus is slowing its contractions, blood may sit in the folds of the uterus or in the vaginal canal longer before making its way out, resulting in more time to oxidize and darken the colour.
Other variables like hormone levels, infections, and period product choice may affect menstrual blood colour as well. For example, those using pads may see darker hues as the blood has sat outside the body oxidizing longer, while those using a cup may find their blood is brighter as it hasn’t had as much oxygen exposure.
Let’s talk about some common colours you may see throughout your cycle.
Bright red menstrual blood indicates fresh, new, period blood. The brightness of red tells us that the uterine lining was recently produced and shed, not giving it time to oxidize into a darker colour. Bright red blood is seen on the first few days of the menstrual phase typically.
Dark maroon hues of menstrual blood are common during the second half of your period or in people with moderate period flows. This blood has had a bit more time in the uterus, being exposed to more oxygen than fresh blood in the early days of the week.
Brown or black menstrual blood indicates the oldest type of menstrual blood and is often seen at the end of the period or spotting. This blood may be coming from the deeper layers of the endometrial lining. People with irregular cycles, or those who don’t get periods often may find they see this colour more often as the blood has been in the uterus longer.
When yellow period blood is seen it’s likely a mix of a bit of menstrual blood with normal cervical fluid (discharge). This can often be seen before your period starts or after. It’s only cause for concern if there is a change in your normal texture/smell of your cervical fluid as well, then you should consult a doctor.
If your discharge/spotting is grey this could be a sign of infection such as Bacterial Vaginosis. Consult a doctor if you see grey discharge or grey pieces of tissue within your period.
It’s normal for your period to appear a different colour at the start versus middle of your menstrual week. Some people may even see a different colour of period blood each day.
Colour variants are typical, it’s the amount of blood/pain associated with your cycle that could be cause for concern if you feel it is too much.
When should I see a doctor about my period blood colour?
Ultimately, you know your body best. If there are sudden changes in what colour period blood you usually see, abnormal texture, and amount of blood, it would be good to check in with your doctor.
Tracking your menstrual cycle daily helps to know what “normal” means for you and gives you information to show your doctors should concern ever arise.
- Yang, H., Zhou, B., Prinz, M., & Siegel, D. (2012, October). Proteomic analysis of menstrual blood.
About the Author
Victoria Alexander (she/they) is the face behind The Elephant in the Womb, a space centred around reproductive health education and menstruality. Victoria strives to further stand up for inclusive menstrual equity and actively works with local government to achieve LGBTQ+ centred period and pregnancy care options. Read Victoria's full bio here.