For some women, on some occasions, these contractions begin soon after the woman reports that the orgasm has started and continue at intervals of about one second with initially increasing, and then reducing, intensity. In some instances, the series of regular contractions is followed by a few additional contractions or shudders at irregular intervals. In other cases, the woman reports having an orgasm, but no pelvic contractions are measured at all.Womens orgasms are preceded by erection of the clitoris and moistening of the opening of the vagina. Some women exhibit a sex flush, a reddening of the skin over much of the body due to increased blood flow to the skin. As a woman nears orgasm, the clitoral glans retracts under the clitoral hood, and the labia minora (inner lips) become darker. As orgasm becomes imminent, the outer third of the vagina tightens and narrows, while overall the vagina lengthens and dilates and also becomes congested from engorged soft tissue.Elsewhere in the body, myofibroblasts of the nipple-areolar complex contract, causing erection of the nipples and contraction of the areolar diameter, reaching their maximum at the start of orgasm. A woman experiences full orgasm when her uterus, vagina, anus, and pelvic muscles undergo a series of rhythmic contractions. Most women find these contractions very pleasurable. Researchers from the University Medical Center of Groningen in the Netherlands correlated the sensation of orgasm with muscular contractions occurring at a frequency of 8–13 Hz centered in the pelvis and measured in the anus.
In their 2009 published study, the "coronal planes during perineal contraction and finger penetration demonstrated a close relationship between the root of the clitoris and the anterior vaginal wall". Buisson and Foldès suggested "that the special sensitivity of the lower anterior vaginal wall could be explained by pressure and movement of clitoriss root during a vaginal penetration and subsequent perineal contraction".Supporting a distinct G-spot is a study by Rutgers University, published 2011, which was the first to map the female genitals onto the sensory portion of the brain; brain scans showed that the brain registered distinct feelings between stimulating the clitoris, the cervix and the vaginal wall – where the G-spot is reported to be – when several women stimulated themselves in a functional magnetic resonance (fMRI) machine. "I think that the bulk of the evidence shows that the G-spot is not a particular thing," stated Barry Komisaruk, head of the research findings. "Its not like saying, What is the thyroid gland? The G-spot is more of a thing like New York City is a thing. Its a region, its a convergence of many different structures." Commenting on Komisaruks research and other findings, Emmanuele Jannini, a professor of endocrinology at the University of Aquila in Italy, acknowledged a series of essays published in March 2012 in The Journal of Sexual Medicine, which document evidence that vaginal and clitoral orgasms are separate phenomena that activate different areas of the brain and possibly suggest key psychological differences between women.