Menstruation menarche has changed little in the United States since the 1950s. Menstruation is the most visible phase of the menstrual cycle and its beginning is used as the marker between 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 (an average of 28 days). Perimenopause is when fertility in a female declines, and menstruation occurs less regularly in
Human sexual response cycle Human sexual response cycle The human sexual response cycle is a four-stage model of physiological responses to sexual stimulation, which, in order of their occurrence, are the excitement phase, plateau phase, orgasmic phase and resolution phase. This physiological response model was first formulated by William H. Masters and Virginia E. Johnson, in their 1966 book "Human Sexual Response". Since then, other human sexual response models have been formulated. The excitement phase (also known as the arousal phase or initial excitement phase) is the first stage of the human sexual response cycle, which occurs as a result of physical or mental