cycles. The first day of menstrual bleeding is the date used for the last menstrual period (LMP). The typical length of time between the first day of one period and the first day of the next is 21 to 45 days in young women, and 21 to 31 days in adults. The average length is 28 days; one study estimated it at 29.3 days. The variability of menstrual cycle lengths is highest for women under 25 years of age and is lowest, that is, most regular, for ages 25 to 39 years. The variability increases slightly for women aged 40 to 44 years. Perimenopause is when a woman's fertility declines, and menstruation occurs less regularly in the years leading up to the final menstrual period, when a woman stops menstruating completely and is no longer fertile. The medical definition of menopause is one year without a period and typically occurs between 45 and 55 years in Western countries. Menopause before age 45 is considered premature in industrialized countries. Like the age of menarche, the age of menopause is largely a result of cultural and biological factors. Illnesses, certain surgeries, or medical treatments may cause menopause to occur earlier than it might have otherwise. Bleeding The average volume of menstrual fluid during a monthly menstrual period is with considered typical. Menstrual fluid is the correct name for the flow, although many people prefer to refer to it as menstrual blood. Menstrual fluid is reddish-brown, a slightly darker color than venous blood. About half of menstrual fluid is blood. This blood contains sodium, calcium, phosphate, iron, and chloride, the extent of which depends on the woman. As well as blood, the fluid consists of cervical mucus, vaginal secretions, and endometrial tissue. Vaginal fluids in menses mainly contribute water, common electrolytes, organ moieties, and at least 14 proteins, including glycoproteins. Many women and girls notice blood clots during menstruation. These appear as clumps of blood that may look like tissue. If there was a miscarriage or a stillbirth, examination under a microscope can confirm if it was endometrial tissue or pregnancy tissue (products of conception) that was shed. Sometimes menstrual clots or shed endometrial tissue is incorrectly thought to indicate an early-term miscarriage of an embryo. An enzyme called plasmin – contained in the endometrium – tends to inhibit the blood from clotting. The amount of iron lost in menstrual fluid is relatively