form the labia minora and the labioscrotal swellings form the labia majora. at this time the sexes still cannot be distinguished. the appearance of the external genitalia is similar in male and female embryos until the twelfth week and even then is difficult to distinguish. the uterovaginal canal or genital canal, forms in the third month of the development of the urogenital system. the lower part of the canal is blocked off by a plate of tissue, the vaginal plate. this tissue develops and lengthens during the third to fifth months and the lower part of the vaginal canal is formed by a process of desquamation or cell shedding. the end of the vaginal canal is blocked off by an endodermal membrane which separates the opening from the vestibule. in the fifth month the membrane degenerates but leaves a remnant called the hymen. organs in the male and female with a shared common ancestry are said to be homologous. the clitoral glans is homologous to the male glans penis, and the clitoral body and the clitoral crura are homologous to the corpora cavernosa of the penis. the labia majora is homologous to the scrotum; the clitoral hood is homologous to the foreskin, and the labia minora is homologous to the spongy urethra. the vestibular bulbs beneath the skin of the labia minora are homologous to the corpus spongiosum, the tissue of the penis surrounding the urethra, and to the bulb of the penis. bartholin's glands are homologous to the bulbourethral glands in males. childhood the newborn's vulva may be swollen or enlarged as a result of having been exposed, via the placenta, to her mother's increased levels of hormones. the labia majora are closed. these changes disappear over the first few months. during childhood before puberty, the lack of estrogen can cause the labia to become sticky and to ultimately join firmly together. this condition is known as labial fusion and is rarely found after puberty when oestrogen production has increased. puberty puberty is the onset of the ability to reproduce, and takes place over two to three years, producing a number of changes. the structures of the vulva become proportionately larger and may become more pronounced. pubarche, the first appearance of pubic hair develops, firstly on the labia majora, and later spreads to the mons pubis, and sometimes to the inner thighs and perineum. pubic hair is much coarser than other body hair, and is considered a secondary sex characteristic. pubarche can occur independently of puberty. premature pubarche may sometimes indicate a later metabolic-endocrine disorder seen at adolescence. the disorder sometimes known as a polyendocrine disorder is marked by elevated levels of androgen, insulin, and lipids, and may originate in the fetus. instead of being seen as a normal variant it is proposed that premature pubarche may be seen as a marker for these later endocrine disorders. apocrine sweat glands secrete sweat into the pubic hair follicles. this is broken down by bacteria on the skin and produces an odor, which some consider to act as an attractant sex pheromone. the labia minora may grow more prominent and undergo changes in color. at puberty the first monthly period known as menarche marks the onset of menstruation. in prepubertal girls the skin of the vulva is thin and delicate, and its neutral ph makes it prone to irritation. the production of the female sex hormone estradiol (an estrogen) at puberty, causes the perineal skin to thicken by keratinising, and this reduces the risk of infection. estrogen also causes the laying down of fat in the development of the secondary sex characteristics. this contributes to the maturation of the vulva with increases in the size of the mons pubis, and the labia majora and the enlargement of the labia minora. pregnancy in pregnancy the vulva and vagina take on a bluish colouring due to venous congestion. this appears between the eighth and twelfth week and continues to darken as the pregnancy continues. estrogen is produced in large quantities during pregnancy and this causes the external genitals to become enlarged. the vaginal opening and the vagina are also enlarged. after childbirth a vaginal discharge known as lochia is produced and continues for about ten days. menopause during menopause, hormone levels decrease, which causes changes in the vulva known as vulvovaginal atrophy. the decreased estrogen affects the mons, the labia, and the vaginal opening and can cause pale, itchy, and sore skin. other visible changes are a thinning of the pubic hair, a loss of fat from the labia majora, a thinning of the labia minora, and a narrowing of the vaginal opening. this condition has been renamed by some bodies as the genitourinary syndrome of menopause as a more comprehensive term. function and physiology main article: female reproductive system the vulva has a major role to play in the reproductive system. it provides entry to, and protection for the uterus, and the right conditions in terms of warmth and moisture that aids in its sexual and reproductive functions. the external organs of the vulva are richly innervated and provide pleasure when properly stimulated. the mons pubis provides cushioning against the pubic bone during intercourse. a number of different secretions are associated with the vulva, including urine (from the urethral opening), sweat (from the apocrine glands), menses (leaving from the vagina), sebum (from the sebaceous glands), alkaline fluid (from the bartholin's glands), mucus (from the skene's glands), vaginal lubrication from the vaginal wall and smegma. smegma is a white substance formed from a combination of dead cells, skin oils, moisture and naturally occurring bacteria, that forms in the genitalia. in females this thickened secretion collects around the clitoris and labial folds. it can cause discomfort during sexual activity as it can cause the clitoral glans to stick to the hood, and is easily removed by bathing. aliphatic acids known as copulins are also secreted in the vagina. these are believed to act as pheromones. their fatty acid composition, and consequently their odor changes in relation to the stages of the menstrual cycle. sexual arousal non-aroused vulva on the left and a sexually aroused vulva on the right with enlarged and shiny labia minora from vaginal lubrication and vasocongestion the clitoris and the labia minora are both erogenous areas in the vulva. local stimulation can involve the clitoris, vagina and other perineal regions. the clitoris is the human female's most sensitive erogenous zone and generally the primary anatomical source of human female sexual pleasure. sexual stimulation of the clitoris (by a number of means) can result in widespread sexual arousal and, if maintained, can result in orgasm. stimulation to orgasm is optimally achieved by a massaging sensation. sexual arousal results in a number of physical changes in the vulva. during arousal vaginal lubrication increases. vulva tissue is highly vascularised; arterioles dilate in response to sexual arousal and the smaller veins will compress after arousal, so that the clitoris and labia minora increase in size. increased vasocongestion in the vagina causes it to swell, decreasing the size of the vaginal opening by about 30%. the clitoris becomes increasingly erect, and the glans moves towards the pubic bone, becoming concealed by the hood. the labia minora increase considerably in thickness. the labia minora sometimes change considerably in color, going from pink to red in lighter skinned women who have not borne a child, or red to dark red in those that have. immediately prior to an orgasm, the clitoris becomes exceptionally engorged, causing the glans to appear to retract into the clitoral hood. rhythmic muscle contractions occur in the outer third of the vagina, as well as the uterus and anus. contractions become less intense and more randomly spaced as the orgasm continues. the number of contractions that accompany an orgasm vary depending on its intensity. an orgasm may be accompanied by female ejaculation, causing liquid from either the skene's gland or bladder to be expelled through the urethra. the pooled blood begins to dissipate, although at a much slower rate if an orgasm has not occurred. the vagina and vaginal opening return to their normal relaxed state, and the rest of the vulva returns to its normal size, position and color. distance between vagina and clitoral glans the distance of the frenula clitoridis at the clitoral glans from the urinary meatus located in the vaginal opening, the so-called cumd (clitoral-urinary meatus distance), is measured. in the 1920s, marie bonaparte conducted surveys among women to find out whether they get an orgasm during coitus. the causes of lack of sexual arousal were known to be aversion or psychological inhibitions. among the test subjects, there were women who, with a man they loved and desired, "felt the greatest pleasure at certain tender touches", but were nevertheless not sufficiently aroused during coitus. bonaparte examined the distance between the clitoris and the vagina in 200 women. in 69% of the women, the cumd was 1.25 to 2.25 cm, most of whom experienced the sexual act as satisfying. in 10% of the women, the cumd was 2.5 cm, 21% measured 2.75 to 3.5 cm. all the women with a large gap stated that they did not experience satisfying pleasure from penile penetration, although some were very sensitive to "precise stroking by the man". bonaparte concluded from this an anatomical causal connection between a large cumd and "vaginal frigidity" explaining why only women with the clitoral glans close to the vagina (so that it is continuously touched by the penis) were able to experience "the highest sexual pleasure" during coitus. glans clitoridis with small distance to the opening contact of the clitoral glans with the penis with small cumd glans clitoridis with medium distance to the opening lack of contact of the clitoral glans with the penis classification of the 200 women into cumd categories selection of women studied and interviewed a study by carney landis and colleagues in 1940 includes statements to this effect: "on the physical side orgasm capacity is related to clitoris-meatus distance." (landis et al. 1940). in 2011, kim wallen and elisabeth lloyd reviewed bonaparte's research and confirmed an inverse correlation between cumd and orgasm through intercourse. in methods taught since the 1970s by betty dodson, the sexual arousal of the woman during vaginal intercourse is to be ensured by the woman independently stimulating her vulva and clitoris continuously with her hands or possibly with a vibrator. according to kim wallen, the cumd says nothing about a happy sex life, rather that a large gap gives couples an opportunity "to be a bit more inventive in how they have sex." clinical significance irritation irritation and itching of the vulva is called pruritus vulvae. this can be a symptom of many disorders, some of which may be determined by a patch test. the most common cause of irritation is thrush, a fungal infection. vulvovaginal health measures can help to prevent many disorders including thrush. infections of the vagina such as vaginosis and of the uterus may produce vaginal discharge which can be an irritant when it comes into contact with the vulvar tissue. inflammation as vaginitis, and vulvovaginitis can result from this causing irritation and pain. ingrown hairs resulting from pubic hair shaving can cause folliculitis where the hair follicle becomes infected; or give rise to an inflammatory response known as pseudofolliculitis pubis. a less common cause of irritation is genital lichen planus, another inflammatory disorder. a severe variant of this is vulvovaginal-gingival syndrome which can lead to narrowing of the vagina, or vulva destruction. many types of infection and other diseases including some cancers may cause irritation. sexually transmitted infections main article: sexually transmitted infection water warts of molluscum contagiosum pubic hair with crab lice vulvar organs and tissues can become affected by different infectious agents such as bacteria and viruses, or infested by parasites such as lice and mites. over thirty types of pathogen can be sexually transmitted, and many of these affect the genitals. most stis do not produce symptoms or symptoms may be mild and not be indicative of an sti. the practice of safe sex can greatly reduce the risk of infection from many sexually transmitted pathogens. the use of condoms (either male or female condoms) is one of the most effective methods of protection. bacterial infections include: chancroid – characterised