In humans, biological sex is determined by five factors present at birth:
- the number and type of sex chromosomes;
- the type of gonads—ovaries or testicles;
- the sex hormones;
- the internal reproductive anatomy (such as the uterus in females); and
- the external genitalia.
People whose five characteristics are not either all typically male or all typically female at birth are intersex.
Intersex traits are not always apparent at birth; some babies may be born with ambiguous genitals, while others may have ambiguous internal organs (testes and ovaries). Others will not become aware that they are intersex—unless they receive genetic testing—because it does not manifest in their phenotype.