Orgasm is a transient peak sensation of intense pleasure that is accompanied by a number of physiological body changes. In men, orgasm is normally accompanied by ejaculation, which makes the event easily identifiable. In women, however, the achievement of orgasm appears to be less facile than for males and recognizing that it has occurred is often difficult for some women. Objective indicators that orgasm has occurred have been sought for many years. Kinsey proposed "the abrupt cessation of the ofttimes strenuous movements and extreme tensions of the previous sexual activity and the peace of the resulting state" as the most obvious evidence that orgasm had occurred in women. Masters and Johnson described the onset of orgasm as a "sensation of suspension or stoppage." In order to serve as a clear marker of orgasm, however, the indicator must involve a bodily change that is unique to orgasm. This necessarily rules out simple measures like peaks of blood pressure, heart and respiratory rates, or even a woman’s own vocalizations because such events can occur during high levels of sexual arousal that fail to culminate in orgasm.

Remarkably, most of the so-called objective indicators of female orgasm rely on the original, nearly 40-year-old observations and descriptions of Masters and Johnson. They include physiological changes that indicate impending orgasm (prospective), occur during actual orgasm (current), and/or indicate that an orgasm has occurred (retrospective). With regard to prospective changes, during sexual arousal the labia become engorged with blood, increase in size, and undergo vivid color changes. The color changes (light pink to deep red) are presumably due to the changing hemodynamics of the tissue in relation to increased blood flow, tissue congestion, and tissue metabolism (oxygen consumption), indicating the balance between oxygenated (red/pink) and deoxygenated or reduced hemoglobin (blue). Following orgasm, the color of the labia rapidly changes (within 10–15 s) from deep red to light pink. There has been little detailed study of the minora labia apart from the suggested mechanism by which they become lubricated and their increased temperature during sexual arousal has been used as an objective indicator of arousal prior to and after orgasm.

Contractions of the vagina, uterus, and anal sphincter have been proposed as current indicators of orgasm. The resting vagina is a collapsed tube lined with a stratified squamous epithelium, approximating an elongated S-shape in longitudinal section and an H-shape in cross-section. It is anchored amid a bed of powerful, voluntary, striated muscles (pelvic diaphragm, consisting of the pubococcygeus and iliococcygeus muscle). According to Masters and Johnson, contractions recorded in the vagina begin about 2–4 s after the subjective experience of the start of orgasm. They occur in many pre- and postmenopausal women and are due to the activation of the circumvaginal striated muscles which involuntarily contract in about 0.8 s repetitions. This squeezes the outer third of the vagina [designated the "orgasmic platform" by Masters and Johnson] with a force that gradually becomes weaker as the interval between contractions increases. Vaginal rhythmic contractions vary greatly between women in their number and strength, and are dependent on the duration of the orgasm and the strength of the pelvic musculature. Masters and Johnson reported that the stronger the orgasm the greater the number of contractions and, thus indirectly, the longer the duration of orgasm (as each contraction was about 0.8 s apart). However, using physiological (pressure) recordings of the contractions, other researchers have failed to find a link between vaginal contractions and the perceived intensity or duration of the orgasm. Moreover, while Masters and Johnson proposed that vaginal contractions are a definitive sign of orgasm having occurred, other authors have noted that not all women who claim to experience orgasms show vaginal contractions. Uterine contractions have also been proposed as the terminative signal for sexual arousal in multiorgasmic women but too few investigations have assessed orgasmic uterine contractions to make a definitive statement. While voluntary contractions of the anal sphincter can occur during sexual arousal and are sometimes used by women to facilitate or enhance arousal, involuntary contractions occur only during orgasm. Such contractions are more frequently observed during masturbation than during coitus. As with uterine contractions, few studies on anal sphincter contractions during orgasm have been published.

A number of questionnaire studies have reported that orgasm through stimulation of the so-called G-spot (named after Ernst Grafenberg, who reportedly first described the phenomenon) causes a substantial number of women to expel fluid from their urethra. However, there has been no scientific evidence to support the assertion that women ejaculate a fluid distinguishable from urine at the time of orgasm. Moreover, there has not been consistent evidence for any anatomical structure or "spot" on the anterior vaginal wall apart from the known paraurethral glands and spongiosal tissue around the urethra, which could cause sexually pleasurable sensations when stimulated.

Physiological changes noted to occur after orgasm (retrospective) include areolae (the pigmented skin area around the nipple of the breasts) decongestion, enhanced vaginal pulse amplitude (measured by photoplethysmography), and raised prolactin levels. During sexual arousal, the primary areolae swell up, likely due to both vasocongestion and smooth muscle contraction. The volume expansion can become so marked that the swollen areolae hide a large part of the base of the erect nipples making it look as though they have lost their erection. At orgasm, the loss of volume is so rapid that the areolae become corrugated before becoming flatter. This provides a visual indicator that orgasm has occurred. In the absence of orgasm, the areolae detumescence is much slower and the corrugation does not develop. There has been minimal study of areolae changes during arousal and orgasm.

Changes in the blood supply to vaginal tissue before, during, and after orgasm were recorded by photoplethysmography in seven young women by Geer and Quartararo. Sexual arousal by masturbation caused an increase in the vaginal pulse amplitude signal compared to the basal values in all women. Immediately after the end of orgasm, however, vaginal pulse amplitude was actually significantly greater than before orgasm in five of the seven women (71%) and was not significantly less in the other two. The postorgasmic period of maximum amplitude lasts for about 10–30 s and then slowly returns to its resting level. Other recordings in the literature have shown similar changes.

Studies by Exton and colleagues have reported that prolactin secretion (a peptide hormone secreted by the lactotrophic cells of the anterior pituitary gland) is not activated by sexual arousal per se but is specifically activated and doubled in plasma concentration with orgasm. This elevation occurs directly after orgasm and is maintained for about 60 min.

In summary, specific physiological indicators that orgasm has occurred include rapid color changes of the labia; contractions of the vagina, uterus, and anal sphincter; areolae decongestion; enhanced vaginal pulse amplitude; and raised prolactin levels.