The fluid that is expelled when a woman is having her period. This is often referred to as “period blood” and it is said that a woman is “bleeding” during her period, but less than half of period flow is made up of blood. The lion’s share of menstrual flow originates in the uterus and passes from there through the small opening in the cervix that is called the Os. It then leaves a woman’s body through her vagina. Menstrual flow is composed of endometrial tissue, blood, and immune cells. The endometrium or wall of the uterus becomes thicker as ovulation approaches in order to supply an embryo with blood and nutrients. If there is no embryo to implant, the thick wall sheds and becomes the main source of a woman’s period flow. Menstrual flow also contains cervical mucus, vaginal secretions, and blood. (The vaginal secretions in period flow are not there because the walls of the vagina shed and bleed, as some people mistakenly believe happens during a woman’s period. The vagina secretions are simply normal fluids that the vagina makes.) The first and second days of a woman’s period are usually when her period flow is the heaviest. Period flow tends to be brighter red during this time, and darker toward the end of a woman’s period because the flow is lighter and it takes longer to exit her body, giving it more time to oxidize and attain a darker color. Periods can generally last from 2 to 8 days, with 3 to 5 days being common. The total amount of period flow is often between 2 to 3 tablespoons for a woman’s entire period, but can range from 5 ml to 80 ml or 90 ml (a teaspoon to almost 6 tablespoons). If a woman wants to know the exact volume of her period flow, the best way to find out is for her to use a menstrual cup. Period flow will often contain small clumps of tissue from the endometrium, which is what is being swept away because conception did not occur. It can also contain clots of blood, especially in the mornings after it might have spent longer in a woman’s vagina. This is usually normal and healthy. Menstrual flow has very little smell, and if it does, it’s probably because it’s been sitting in a pad or tampon for too long. When a woman is using hormonal methods of birth control, her endometrium tends to be flatter and not nearly as thick. This is why her period flow might be lighter or there is barely any flow at all if she is using hormonal methods of birth control. Unless a woman has a blood-borne infection, there is nothing about period flow that is harmful to a male partner. If she feels like having intercourse during her period, there is no reason for her not to unless she has a medical condition that precludes her from having intercourse.