Menstruation is a woman's monthly bleeding. It is also called menses, menstrual period, or period. When a woman has her period, she is menstruating. The menstrual blood is partly blood and partly tissue from the inside of the uterus. It flows from the uterus through the small opening in the cervix, and passes out of the body through the vagina. Most menstrual periods last from 3 to 5 days.
In the first half of the menstrual cycle, levels of estrogen rise and make the lining of the uterus grow and thicken. In response to follicle-stimulating hormones, an egg in one of the ovaries starts to mature. At about day 14 of a typical 28-day cycle, in response to a surge of luteinizing hormone, the egg leaves the ovary. This is called ovulation.
In the second half of the menstrual cycle, the egg begins to travel through the fallopian tube to the uterus. Progesterone levels rise and help prepare the uterine lining for pregnancy. If the egg becomes fertilized by a sperm cell and attaches itself to the uterine wall, the woman becomes pregnant. If the egg is not fertilized, it either dissolves or is absorbed into the body. If pregnancy does not occur, estrogen and progesterone levels drop, and the thickened lining of the uterus is shed during the menstrual period.