Intersex people are individuals born with any of several sex characteristics including chromosome patterns, gonads, or genitals that, according to the Office of the United Nations High Commissioner for Human Rights, "do not fit typical binary notions of male or female bodies".

Sex assignment at birth usually aligns with a child's anatomical sex and phenotype. The number of births with ambiguous genitals is in the range of 0.02% to 0.05%. Other conditions involve atypical chromosomes, gonads, or hormones. Some persons may be assigned and raised as a girl or boy but then identify with another gender later in life, while most continue to identify with their assigned sex. The number of births where the baby is intersex has been reported differently depending on who reports and which definition of intersex is used. Anne Fausto-Sterling and her co-authors suggest that the prevalence of "nondimorphic sexual development" might be as high as 1.7%.

A study published by Leonard Sax reports that this figure includes conditions which most clinicians do not recognize as intersex, and that in those "conditions in which chromosomal sex is inconsistent with phenotypic sex, or in which the phenotype is not classifiable as either male or female", the prevalence of intersex is about 0.018%.

Terms used to describe intersex people are contested, and change over time and place. Intersex people were previously referred to as "hermaphrodites" or "congenital eunuchs". In the 19th and 20th centuries, some medical experts devised new nomenclature in an attempt to classify the characteristics that they had observed, the first attempt to create a taxonomic classification system of intersex conditions. Intersex people were categorized as either having "true hermaphroditism", "female pseudohermaphroditism", or "male pseudohermaphroditism".

These terms are no longer used, and terms including the word "hermaphrodite" are considered to be misleading, stigmatizing, and scientifically specious in reference to humans. In biology, the term "hermaphrodite" is used to describe an organism that can produce both male and female gametes. Some people with intersex traits use the term "intersex", and some prefer other language.

In clinical settings, the term "disorders of sex development" (DSD) has been used since 2006, a shift in language considered controversial since its introduction.

Intersex people face stigmatization and discrimination from birth, or following the discovery of intersex traits at stages of development such as puberty. Intersex people may face infanticide, abandonment, and the stigmatization of their families.

Globally, some intersex infants and children, such as those with ambiguous outer genitalia, are surgically or hormonally altered to create more socially acceptable sex characteristics. However, this is considered controversial, with no firm evidence of favorable outcomes. Such treatments may involve sterilization. Adults, including elite female athletes, have also been subjects of such treatment. Increasingly, these issues are considered human rights abuses, with statements from international and national human rights and ethics institutions.

Klinefelter syndrome (KS), also known as 47,XXY, is a syndrome where a male has an additional copy of the X chromosome. The primary features are infertility and small, poorly functioning testicles. Usually, symptoms are subtle and subjects do not realize they are affected. Sometimes, symptoms are more evident and may include weaker muscles, greater height, poor motor coordination, less body hair, breast growth, and less interest in sex. Often, these symptoms are noticed only at puberty. Intelligence is usually normal, but reading difficulties and problems with speech are more common. Hypospadias is a common variation in fetal development of the penis in which the urethra does not open from its usual location in the head of the penis. It is the second-most common birth abnormality of the male reproductive system, affecting about one of every 250 males at birth.[3] Roughly 90% of cases are the less serious distal hypospadias, in which the urethral opening (the meatus) is on or near the head of the penis (glans). The remainder have proximal hypospadias, in which the meatus is all the way back on the shaft of the penis, near or within the scrotum. Shiny tissue that should have made the urethra extends from the meatus to the tip of the glans; this tissue is called the urethral plate.

In most cases, the foreskin is less developed and does not wrap completely around the penis, leaving the underside of the glans uncovered. Also, a downward bending of the penis, commonly referred to as chordee, may occur. Chordee is found in 10% of distal hypospadias and 50% of proximal hypospadias cases at the time of surgery. Also, the scrotum may be higher than usual on either side of the penis (called penoscrotal transposition).

The cause of hypospadias is unknown. It most often occurs by itself, without other variations, although in about 10% of cases it may be part of an intersex condition or a medical syndrome with multiple abnormalities.

The most common associated difference is an undescended testicle, which has been reported in around 3% of infants with distal hypospadias and 10% with proximal hypospadias. The combination of hypospadias and an undescended testicle sometimes indicates a child has an intersex condition, so additional testing may be recommended to make sure the child does not have congenital adrenal hyperplasia with salt wasting or a similar condition where immediate medical intervention is needed. Otherwise no blood tests or X-rays are routinely needed in newborns with hypospadias.

Turner syndrome (TS), also known as 45,X, or 45,X0, is a genetic condition in which a female is partially or completely missing an X chromosome. Signs and symptoms vary among those affected. Often, a short and webbed neck, low-set ears, low hairline at the back of the neck, short stature, and swollen hands and feet are seen at birth. Typically, those affected do not develop menstrual periods and breasts without hormone treatment and are unable to have children without reproductive technology. Heart defects, diabetes, and low thyroid hormone occur in the disorder more frequently than average. Most people with TS have normal intelligence; however, many have problems with spatial visualization that may be needed in order to learn mathematics. Vision and hearing problems also occur more often than average.

Vaginal atresia is a condition in which the vagina is abnormally closed or absent. The main causes can either be complete vaginal hypoplasia, or a vaginal obstruction, often caused by an imperforate hymen or, less commonly, a transverse vaginal septum. It results in uterovaginal outflow tract obstruction. This condition does not usually occur by itself within an individual, but coupled with other developmental disorders within the female. One out of every 5,000 women have this abnormality.

XYY syndrome, also known as Jacobs syndrome, is an aneuploid genetic condition in which a male has an extra Y chromosome. There are usually few symptoms. These may include being taller than average, acne, and an increased risk of learning disabilities. The person is generally otherwise normal, including typical rates of fertility.

True hermaphroditism, sometimes referred to as ovotesticular syndrome, is an intersex condition in which an individual is born with both ovarian and testicular tissue. Commonly, one or both gonads is an ovotestis containing both types of tissue.

Androgen insensitivity syndrome (AIS) is a difference in sex development involving hormonal resistance due to androgen receptor dysfunction.

It affects 1 in 20,000 to 64,000 XY (karyotypically male) births. The condition results in the partial or complete inability of cells to respond to androgens. This unresponsiveness can impair or prevent the development of male genitals, as well as impairing or preventing the development of male secondary sexual characteristics at puberty. It does not significantly impair female genital or sexual development.[4] The insensitivity to androgens is therefore clinically significant only when it occurs in genetic males, (i.e. individuals with a Y-chromosome, or more specifically, an SRY gene). Clinical phenotypes in these individuals range from a typical male habitus with mild spermatogenic defect or reduced secondary terminal hair, to a full female habitus, despite the presence of a Y-chromosome.

AIS is divided into three categories that are differentiated by the degree of genital masculinization: complete androgen insensitivity syndrome (CAIS) is indicated when the external genitalia are those of a typical female; mild androgen insensitivity syndrome (MAIS) is indicated when the external genitalia are those of a typical male, and partial androgen insensitivity syndrome (PAIS) is indicated when the external genitalia are partially, but not fully, masculinized. Androgen insensitivity syndrome is the largest single entity that leads to 46,XY undermasculinized genitalia.

Anorchia (also called anorchidism or anorchism) is a disorder of sex development in which a person with XY karyotype, which usually corresponds to male sex, is born without testes. Within a few weeks of fertilization, the embryo develops rudimentary gonads (testes), which produce hormones responsible for the development of the reproductive system. If the testes fail to develop within eight weeks, the baby will develop female genitalia. If the testes begin to develop but are lost or cease to function between eight and 10 weeks, the baby will have ambiguous genitalia when it is born. However, if the testes are lost after 14 weeks, the baby will have partial male genitalia with the notable absence of gonads.

Tests include observable lack of testes, low testosterone levels (typical female levels), elevated follicle stimulating hormone and luteinizing hormone levels, XY karyotype, ultrasound or magnetic resonance imaging showing absent gonadal tissue, low bone density, low anti-Müllerian hormone levels, and surgical exploration for evidence of male gonadal tissue.

Persistent Müllerian duct syndrome (PMDS) is the presence of Müllerian duct derivatives (fallopian tubes, uterus, and/or the upper part of the vagina) in what would be considered a genetically and otherwise physically normal male animal by typical human based standards. In humans, PMDS typically is due to an autosomal recessive congenital disorder and is considered by some to be a form of pseudohermaphroditism due to the presence of Müllerian derivatives.

Typical features include undescended testes (cryptorchidism) and the presence of a small, underdeveloped uterus in an XY infant or adult. This condition is usually caused by deficiency of fetal anti-Müllerian hormone (AMH) effect due to mutations of the gene for AMH or the anti-Müllerian hormone receptor, but may also be as a result of insensitivity to AMH of the target organ.

Sexual differentiation Main article: Sexual differentiation The common pathway of sexual differentiation, where a productive human female has an XX chromosome pair, and a productive male has an XY pair, is relevant to the development of intersex conditions.

During fertilization, the sperm adds either an X (female) or a Y (male) chromosome to the X in the ovum. This determines the genetic sex of the embryo. During the first weeks of development, genetic male and female fetuses are "anatomically indistinguishable", with primitive gonads beginning to develop during approximately the sixth week of gestation. The gonads, in a bipotential state, may develop into either testes (the male gonads) or ovaries (the female gonads), depending on the consequent events.[179] Up until and including the seventh week, genetically female and genetically male fetuses appear identical.

At around eight weeks of gestation, the gonads of an XY embryo differentiate into functional testes, secreting testosterone. Ovarian differentiation, for XX embryos, does not occur until approximately week 12 of gestation. In typical female differentiation, the Müllerian duct system develops into the uterus, Fallopian tubes, and inner third of the vagina. In males, the Müllerian duct-inhibiting hormone MIH causes this duct system to regress. Next, androgens cause the development of the Wolffian duct system, which develops into the vas deferens, seminal vesicles, and ejaculatory ducts.[179] By birth, the typical fetus has been completely sexed male or female, meaning that the genetic sex (XY-male or XX-female) corresponds with the phenotypical sex; that is to say, genetic sex corresponds with internal and external gonads, and external appearance of the genitals.

Signs

• There are a variety of symptoms that can occur. Ambiguous genitalia is the most common sign. There can be micropenis, clitoromegaly, partial labial fusion, electrolyte abnormalities, delayed or absent puberty, unexpected changes at puberty, hypospadias, labial or inguinal (groin) masses (which may turn out to be testes) in girls and undescended testes (which may turn out to be ovaries) in boys.

Ambiguous genitalia

• Ambiguous genitalia may appear as a large clitoris or as a small penis.

The Quigley scale is a method for describing genital development in AIS.

• Because there is variation in all of the processes of the development of the sex organs, a child can be born with a sexual anatomy that is typically female or feminine in appearance with a larger-than-average clitoris (clitoral hypertrophy) or typically male or masculine in appearance with a smaller-than-average penis that is open along the underside.
• The appearance may be quite ambiguous, describable as female genitals with a very large clitoris and partially fused labia, or as male genitals with a very small penis, completely open along the midline ("hypospadic"), and empty scrotum. Fertility is variable.[citation needed]

Measurement systems for ambiguous genitalia

• The orchidometer is a medical instrument to measure the volume of the testicles. It was developed by Swiss pediatric endocrinologist Andrea Prader. The Prader scale and Quigley scale are visual rating systems that measure genital appearance.

Other signs

• In order to help in classification, methods other than a genitalia inspection can be performed. For instance, a karyotype display of a tissue sample may determine which of the causes of intersex is prevalent in the case. Additionally, electrolyte tests, endoscopic exam, ultrasound and hormone stimulation tests can be done.

Causes

• Intersex can be divided into four categories which are: 46, XX intersex; 46, XY intersex; true gonadal intersex; and complex or undetermined intersex.

46, XX intersex

• This condition used to be called "female pseudohermaphroditism". Persons with this condition have female internal genitalia and karyotype (XX) and various degree of external genitalia virilization. External genitalia is masculinized congenitally when female fetus is exposed to excess androgenic environment.
• Hence, the chromosome of the person is of a woman, the ovaries of a woman, but external genitals that appear like a male. The labia fuse, and the clitoris enlarges to appear like a penis. The causes of this can be male hormones taken during pregnancy, congenital adrenal hyperplasia, male-hormone-producing tumors in the mother and aromatase deficiency.

46, XY intersex

• This condition used to be called "male pseudohermaphroditism". This is defined as incomplete masculinization of the external genitalia. Thus, the person has the chromosomes of a man, but the external genitals are incompletely formed, ambiguous, or clearly female. This condition is also called 46, XY with undervirilization. 46, XY intersex has many possible causes, which can be problems with the testes and testosterone formation.
• Also, there can be problems with using testosterone. Some people lack the enzyme needed to convert testosterone to dihydrotestosterone, which is a cause of 5-alpha-reductase deficiency. Androgen insensitivity syndrome is the most common cause of 46, XY intersex.

True gonadal intersex

• This condition used to be called "true hermaphroditism". This is defined as having asymmetrical gonads with ovarian and testicular differentiation on either sides separately or combined as ovotestis. In most cases, the cause of this condition is unknown.

Complex or undetermined intersex

• This is the condition of having any chromosome configurations rather than 46, XX or 46, XY intersex. This condition does not result in an imbalance between internal and external genitalia. However, there may be problems with sex hormone levels, overall sexual development, and altered numbers of sex chromosomes.[