Penis & Size

Measuring the penis
To get the most accurate measurements for an individual's penis size, it has been recommended that several measurements be taken at different times, preferably with different erections on different days. Then average these figures together. This is to account for what may be natural variability in size due to factors such as arousal level, time of day, room temperature, frequency of sexual activity, and unreliability of the measurement methods.

Length
The length can be measured with the subject standing and the penis held parallel to the floor. The penis is measured along the top, from the base to the tip. Results are inaccurate if the measurement is taken along the underside of the penis, or if the subject is seated or prone.

Circumference
Penis girth is the measurement of the circumference of the completely erect penis. It is variously quoted as an average of three measurements: just below the glans penis, in the middle of the shaft, and at the base.

Problems
A penis must be completely erect in order to obtain an accurate measurement. This may be difficult to achieve in a clinical setting. At least one Brazilian doctor resorted to injecting penises with drugs to induce erection, which gave more consistent results. Some clinicians measure the penis by stretching the flaccid penis as far as comfortably possible. Self-reported measurements tend to be unreliable because men often want to report a larger penis size.

Studies on penis size
While results vary across studies, the consensus is that the average human penis is approximately 12.9–15 cm (5.1–5.9 in) in length with a 95% confidence interval of (10.7 cm, 19.1 cm) (or, equivalently, 4.23 in, 7.53 in). The typical girth or circumference is approximately 12.3 cm (4.85 in) when fully erect. The average penis size is slightly larger than the median size (or, put another way, most penises are below average in size).

Size at birth
The average stretched penile length at birth is about 4 cm (1.6 in), and 90% of newborn boys will be between 2.4 and 5.5 cm (0.9 and 2.2 in). Limited growth of the penis occurs between birth and 5 years of age, but very little occurs between 5 years and the onset of puberty. The average size at the beginning of puberty is 6 cm (2.4 in) with adult size reached about 5 years later. W.A. Schonfeld published a penis growth curve in 1943.

Size with aging
Age is not believed to negatively correlate with penis size. “Individual research studies have ... suggested that penis size is smaller in studies focusing on older men, but Wylie and Eardley found no overall differences when they collated the results of various studies [over a 60 year period].”

Erect length
Several scientific studies have been performed on the erect length of the adult penis. Studies which have relied on self-measurement, including those from Internet surveys, consistently reported a higher average length than those which used medical or scientific methods to obtain measurements.
The following staff-measured studies are each composed of different subgroups of the human population (i.e. specific age range and/or race; selection of those with sexual medical concerns or self-selection) which could cause a sample bias.
A study published in the September 1996 Journal of Urology concluded that average erect length was 12.9 cm (5.08 in) (measured by staff). The purpose of the study was to “provide guidelines of penile length and circumference to assist in counseling patients considering penile augmentation.” Erection was pharmacologically induced in 80 physically normal American men (varying ethnicity, average age 54). It was concluded: “Neither patient age nor size of the flaccid penis accurately predicted erectile length.”
A study published in the December 2000 International Journal of Impotence Research found that average erect penis length in 50 Jewish Caucasian males was 13.6 cm (5.35 in) (measured by staff). Quote: "The aim of this prospective study was to identify clinical and engineering parameters of the flaccid penis for prediction of penile size during erection." Erection was pharmacologically induced in 50 Jewish Caucasian patients who had been evaluated for Erectile dysfunction (average age 47±14y). Patients with penis abnormalities or whose ED could be attributed to more than one psychological origin were omitted from the study.
A study conducted by LifeStyles Condoms found an average of 14.9 cm (5.9 in) with a standard deviation of 2.1 cm (0.8 in) (measured by staff). The purpose of this study was to ensure properly sized condoms were available. 401 college students volunteered to be measured during 2001 Spring Break in Cancún, Mexico, of which 300 gained an erection to be clinically measured (without pharmacological aid). The 300 effective measurements makes this study double the size of any prior study that uses medical staff to measure penis size (but the sample is more representative of male American college students visiting Cancun than it is of the entire male American population).

Erect circumference
The Cancún LifeStyle study found an average of 12.6 cm (5.0 in) with a standard deviation of 1.3 cm (0.5 in).
Similar results exist regarding studies of the circumference of the adult fully erect penis, with the measurement taken mid-shaft.[citation needed] As with length, studies that relied on self-measurement consistently reported a higher average than those with staff measuring.

Flaccid length
One study found the mean flaccid penis length to be 3.5 inches (8.9 cm) (measured by staff). Length of the flaccid penis does not necessarily correspond to length of the erect penis; some smaller flaccid penises can grow much larger, and some larger flaccid penises cannot grow much larger.
The penis and scrotum can contract involuntarily in reaction to cold temperatures or nervousness, referred to by the slang term "shrinkage", due to action by the cremaster muscle. The same issue affects cyclist and exercise bike users, with prolonged pressure on the perineum from the saddle, and the straining of the exercise causing the penis and scrotum to contract involuntarily which is sometimes referred to as "gym balls" or "saddle balls". An incorrect saddle may ultimately cause erectile dysfunction.

Penis size and female genital response
According to some sex researchers and therapists, several misconceptions have developed surrounding penile-vaginal intercourse. Many men exaggerate the importance of deep vaginal penetration in stimulating a woman to orgasm.
The most sensitive area of the female genitals includes the vulva, clitoris, and the section of vagina closest to the outside of a woman's body, which is roughly 10 centimeters (4 in) in length. Research has found that portions of the clitoris extend into the vulva and vagina. Given that the median penis size is above this length, the majority of penises are of sufficient length to fully satisfy their partners.
While many women find penile stimulation of the cervix to be uncomfortable or painful, others report it to be the key to orgasm. The cervix may be confused with the anterior or posterior fornix, the deepest point of the vagina, above and below the cervix, respectively. The cervix and fornix are close to each other, making it possible for there to be indirect and/or simultaneous stimulation between them.
The fornix is said to be another possible orgasm trigger area. Tests have shown that pressure on this area causes the vagina to lubricate very quickly. The area of sexual response in the anterior fornix has also been called the epicentre, T-Spot, AFE-Zone, AFE or A-Spot; while in the posterior fornix it has been called epicenter (as well) or cul-de-sac (since the cul-de-sac, also known as the rectouterine pouch, may be indirectly stimulated by pressure on the posterior fornix).
During arousal, the vagina lengthens rapidly to an average of about 4 in (8.5 cm), but can continue to lengthen in response to pressure. As the woman becomes fully aroused, the vagina tents (last ²⁄₃ expands in length and width) while the cervix retracts. The walls of the vagina are composed of soft elastic folds of mucous membrane skin which stretch or contract (with support from pelvic muscles) to the size of the penis. This means (with proper arousal) the vagina stretches or contracts to accommodate virtually any size penis, from small to large.

Penis enlargement surgery
Main article: Penis enlargement
Surgical techniques used for penis enlargement (enhancement phalloplasty) -- penis lengthening and penile widening (girth enhancement) -- have been in the urologic literature for many years.
There are two basic ways of enlarging the penis:
Penis lengthening surgery involves the release of the fundiform ligament and the suspensory ligament that attaches the 2 erectile bodies to the pubic bone. Once these ligaments have been cut, part of the penile shaft (usually held within the body) drops forward and extends out, enlarging the penis in 2-3 cm.
Penis widening with PMMA injection. This method involves injection of silicone, PMMA and other materials into the penis and scrotum, to achieve girth enlargement.