Sex assignment typically happens at birth based on anatomical and physiological markers.
Male and female genitalia, both internal and external, are different, and male and female bodies have distinct hormonal and chromosomal makeups. Doctors use these factors to assign natal sex.
At birth, female-assigned people have higher levels of estrogen and progesterone, and while assigned males have higher levels of testosterone. Assigned females typically have two copies of the X chromosome, and assigned males have one X and one Y chromosome.
Society often sees maleness and femaleness as a biological binary. However, there are issues with this distinction. For instance, the chromosomal markers are not always clear-cut. Some male babies are born with two or three X chromosomes, just as some female babies are born with a Y chromosome.
Also, some babies are born with atypical genitalia due to a difference in sex development.
This type of difference was once called a “disorder of sex development,” but this term is problematic. In a 2015 surveyTrusted Source, most respondents perceived the term negatively. A further review found that many people do not use it at all, and instead use “intersex.”
Being intersex can mean different things. For example, a person might have genitals or internal sex organs that fall outside of typical binary categories. Or, a person might have a different combination of chromosomes. Some people do not know that they are intersex until they reach puberty.
Biologists have started to discussTrusted Source the idea that sex may be a spectrum. This is not a new concept but one that has taken time to come into the public consciousness. For example, the idea of sex as a spectrum was discussed in a 1993 article published by the New York Academy of Sciences.