During menstruation, the body sheds tissue and blood from the uterus through the vagina. This bloody discharge can vary from bright red to dark brown or black depending on how old it is. Blood that stays in the uterus long enough will react with oxygen (oxidize). Blood that has had time to oxidize appears darker. Hormonal changes and health conditions can also affect the color and texture of period blood.

Blood (and tissue) changes color depending on how long it has been exposed to air oxygenation. Think of when you cut your skin—red blood comes out of a fresh wound. If you put a bandage on it and check it the next day, you’ll see that once-red blood will have turned brown. Blood color appears darker because it has reacted with oxygen, and the majority of the water in blood will have evaporated, making a more concentrated pigmentation.

Your endometrium is the inner lining of your uterus, which is where a fertilized egg would implant and grow. The endometrium is made up of highly vascularized tissue with special spiralized arteries. This provides a fertilized egg with quick and easy access to a fresh blood supply (carrying nutrients and oxygen), so it can start to develop.

Right before you get your period, these specialized spiral arteries constrict, to limit blood loss. After the constriction of the spiral arteries, the endometrium starts to break away in pieces from the deeper layers of the uterus. Your endometrium does not separate all at once, it’s a slower, controlled separation, and it takes time for your endometrial tissue to make its way down through your cervix  and vagina. This initial blood and tissue may appear dark red or brown, or even black because it takes longer to exit your body.

Brown/Black

This can be more commonly seen at the beginning or end of your period. Experiencing a brown/black flow is due to oxidized blood that is left in the uterus from your previous menstrual cycle (old blood) and is taking longer to leave your uterus. This can be commonly seen with regular tampon or oral contraceptive use and common postpartum.

Brown/black flow can be a telltale sign of having low progesterone levels. Progesterone is the dominant hormone in the second half (luteal phase) of your menstrual cycle. Symptoms of low progesterone levels include headaches or irritability before period, premenstrual syndrome (PMS), miscarriage in the first trimester, anxiety/depression, restless legs at night, difficulty trying to conceive, and irregular periods.

Brown/black flow can be a sign of a vaginal infection. Symptoms of vaginal infection include vaginal itching, abdominal pain, burning on urination, spotting throughout the cycle, a foul smell, and brown with yellow/grey discharge. Seek medical attention if you are experiencing any of these symptoms.

Black blood can indicate a vaginal blockage.

Brown blood can be a sign of implantation bleeding and an early sign of pregnancy. Or postpartum bleeding called lochia, which is vaginal discharge that occurs after giving birth. Brown bleeding can also be a common symptoms of polycystic ovarian syndrome (PCOS).

Dark Red/Purple

Dark red flow is also old blood that has been left in the uterus from your previous menstrual cycle, but not as old as brown/black blood. Dark red flow can be seen with heavy periods due to high estrogen levels or an underlying iron deficiency. Purple color in period blood can also be a sign of excess estrogen levels and may show up as large blood clots (larger than the size of a quarter).

Estrogen is the most dominant hormone during the first half (follicular phase) of your menstrual cycle. High levels of estrogen can cause symptoms of heavy menstrual bleeding, blood clots, breast tenderness, endometriosis, PCOS, ovarian cysts and emotional PMS.

Pink

Pink colored blood can be a sign of poor blood quality, from having low blood cell production and/or nutrient deficiencies. Pink blood can specifically be seen with nutritional anemias (iron, folate, or B12 deficiencies). Blood helps to nourish every organ in the body and our reproductive tract. A lack of nutrients and oxygen flow to the uterus creates a hostile environment for embryo implantation or development. Symptoms of nutritional anemias: missed/heavy menstrual cycle, fatigue, muscle weakness, or numbness sensations.

Bleeding that mixes with fertile cervical fluid can appear light pink. Mid-cycle pink spotting may just be a sign that you’re ovulating. The use of hormonal birth control can lower estrogen levels, also leading to light pink color blood during your period. Pink blood could be caused by lower estrogen levels thanks to perimenopause or primary ovarian insufficiency.

Bright Red

Bright red blood is a normal, optimal flow color to see during your period. It’s especially healthy if you have a regular menstrual cycle. When your uterine lining sheds faster, the blood is brighter in color and is newer blood, meaning the blood doesn’t have time to oxidize if it was left in the uterus for longer (brown/black blood).

However, if you see bright red blood and it’s not during your period there may be an underlying problem happening. Issues like uterine fibroids, sexually transmitted infections (STI) like gonorrhea and chlamydia, and cervical cancer can cause bright red bleeding between periods. Talk to your doctor if you are experiencing bleeding after sex or between periods.

Grey

This can be a sign of an infection caused by bacterial vaginosis (BV), a condition characterized by an imbalance of vaginal bacteria and a lack of healthy lactobacilli species of bacteria. This causes an overgrowth of bacteria leading to symptoms of grey vaginal discharge and a fishy odor smell, itching and redness. Bacterial vaginosis is easy to treat, talk to your doctor today if you have signs of BV.

Orange

Can also be a color created from period blood mixing with cervical fluid. But orange blood can be a sign of a vaginal infection caused by bacterial vaginosis or trichomoniasis. Bacterial vaginosis and trichomoniasis can be diagnosed with a vaginal culture. Symptoms of trichomoniasis can include a fishy odor more pronounced than bacterial vaginosis, itching, irritation, pain with intercourse, and frothy or foamy discharge.