The genetic sex of a child is determined at conception. The mother's egg cell (ovum) contains an X chromosome, while the father's sperm cell contains either an X or a Y chromosome. These X and Y chromosomes determine the child's genetic sex. Normally, an infant inherits ONE PAIR of sex chromosomes -- one X from the mother and one X or one Y from the father. Thus, it is the father who "determines" the genetic sex of the child. A baby who inherits the X chromosome from the father is a genetic female (two X chromosomes). A baby who inherits the Y chromosome from the father is a genetic male (one X and one Y chromosome).

The male and female reproductive organs and genitals both arise from the same tissue in the fetus. If the process that causes this fetal tissue to become "male" or "female" is disrupted, ambiguous genitalia can develop.

Ambiguous genitalia are those in which it is difficult to classify the infant as male or female. The extent of the ambiguity varies. In very rare instances, the physical appearance may be fully developed as the opposite of the genetic sex (e.g., a genetic male may have developed the appearance of a normal female).

Typically, ambiguous genitalia in genetic females (babies with two X chromosomes) include an enlarged clitoris that has the appearance of a small penis. The urethral opening (where urine comes out) can be anywhere along, above, or below the surface of the clitoris. The labia may be fused, resembling a scrotum. The infant may be thought to be a male with undescended testicles. Sometimes a lump of tissue is felt within the fused labia, further making it look like a scrotum with testicles.

In a genetic male (one X and one Y chromosome), ambiguous genitalia typically include a small penis (less than 2-3 centimeters or 0.8-1.2 inches) that may appear to be an enlarged clitoris (the clitoris of a newborn female is normally somewhat enlarged at birth). The urethral opening may be anywhere along, above, or below the penis; it can be placed as low as on the peritoneum, further making the infant appear to be female. There may be a small scrotum with any degree of separation, resembling labia. Undescended testicles commonly accompany ambiguous genitalia.

Ambiguous genitalia is usually not life threatening (see Causes section for exceptions), but can create social upheaval for the child and the family. For this reason, a team of experienced specialists, including neonatologists, geneticists, endocrinologists, and psychiatrists or social workers will be involved.

• Pseudohermaphroditism -- the genitalia are of one sex, but some physical characteristics of the other sex are present.
• True hermaphrodism -- very rare -- both ovarian and testicular tissue is present and the child may have parts of both male and female genitalia.
• Mixed gonadal dysgenesis -- some cells in the body have only a single X chromosome (female) and some cells in the body have XY chromosomes (male). This syndrome is variable, and depends on which cells are female, which cells are male, and what percent of each there are.
• Congenital adrenal hyperplasia -- has several forms, but the most common form causes the genetic female to appear male. Male infants with the most common form appear normal at birth, but may show premature sexual development as early as 6 months of age. Many states test for this as part of the newborn screen (the blood test your baby has at 24-48 hours of life) because CAH can be life-threatening.
• chromosomal abnormalities, including:
  • Klinefelter's syndrome (XXY) -- male, usually has small testes; causes infertility; may be associated with learning defects or mental retardation; affects 1/500-1/1000 live male births)
  • Turner's syndrome (XO) -- female; usually does not cause ambiguous genitalia (usually looks female at birth), but may have webbed neck, swelling of the hands and feet and other characteristic physical findings at birth; may not be diagnosed until later in life when sexual maturation does not take place; infertile)
• Maternal ingestion of certain medications (particularly androgenic steroids) -- may make a genetic female look more male
• Lack of production of specific hormones, causing the embryo to develop with a female body type regardless of genetic sex
• Lack of testosterone cellular receptors (so even if the body makes the hormones needed to develop into a physical male, the body is unable to respond to those hormones, and therefore, a female body-type is the result even if the genetic sex is male)