When we talk about a person’s sex, we are usually talking about whether that person is male or female from a biological (physical) point of view. The aspects of your sex include your “sex chromosomes” (which we will talk more about in a moment), some of your genes which are not on your “sex chromosomes,” and your sex parts, like your ovaries or testicles, your vagina, clitoris, penis, scrotum, and so on. Sex hormones are another aspect of your sex; sex hormones are the chemical messengers in your body that move through the blood. (Testosterone and estrogen are two examples of sex hormones.) Sex hormones help your body develop and function sexually. For example, before you are born, sex hormones contributed to the development of your genitalia, and during puberty, they helped your body change from a child’s to a man’s or woman’s.

From the time we are conceived until the time we die, our bodies pass through many steps of sex development. For example, during your puberty, you sexually matured, going from having a child’s body to having the body of a sexually mature man or a woman. Puberty, like menopause, is just one obvious stage of sex development. But there are a lot more stages that we do not usually see so clearly. Sex development starts right at conception, goes through the time in the womb, and continues through early childhood, adolescence, adulthood, and late adulthood.

“Sex differentiation” is the term for when boys and girls, or men and women, take different paths of sex development. For example, in the womb, a few weeks after conception, an embryo will form “proto-gonads.” Several weeks later, those proto-gonads will usually take one of two paths to become either testes or ovaries. So that is one time when sex development happens—when, long before birth, the proto-gonads differentiate to become ovaries or testes.

Genitals (penis, clitoris, scrotum, labia, etc.) also differentiate at various stages of human life. Children with DSD sometimes have genitals that look different than the average. Not all children with DSD have genitals that look different than the average, and not all people who have genitals that look different than the average have DSD. (“Averages” are just that—the middle of the spectrum.)

We could say that the first stage of sex differentiation happens right at conception. The egg and the sperm usually each contain 23 chromosomes, little bits of matter that contain genes. Genes are like instructions for building the human body. Along with other chromosomes, usually the egg from the mother contributes one X chromosome, and usually the sperm from the father contributes either one X or one Y chromosome. So we might say the first stage of sex differentiation happens at conception. If an embryo ends up with an XX combination, usually the child that grows from that embryo will become a girl. If the embryo ends up with an XY combination, usually the child that grows from that embryo will become a boy.

Because there are so many stages of sex development in human life, there are lots of opportunities for a person to develop along a path that isn’t the average one for boys or girls. When a less-common path of sex development is taken, the condition is often called a “disorder of sex development” or DSD. So DSD is a name given to a lot of different variations of sex development. Often these conditions are called by a more specific name, like “virilizing congenital adrenal hyperplasia,” or “androgen insensitivity syndrome.”

No one is exactly sure about the answer to this question. There are three reasons for that: (1) There is no central registry for DSD, so we are not sure how many people are diagnosed with them. (2) Sometimes people who are born with DSD are not diagnosed with DSD, so even if we counted everyone who was diagnosed, that would not tell us the real number. (3) Sometimes one set of doctors will count a certain condition as a “disorder of sex development” and another set of doctors will just count it as a variation of sex anatomy. So some doctors will count a large clitoris or a small penis as a DSD while others will not count it that way. There’s not an easy answer to what should count.

We do know this: Between one in 1500 and one in 2000 children are born with noticeable “genital ambiguity,” meaning genitals that do not look like the types common in either females or males. But more people than that have DSD that do not show up at birth because not all people with DSD are born with genitals that look unusual.

It is hard for a lot of people to talk about sex anatomy, especially when it involves children. You may have heard people talk about “hermaphrodites” (a mythological being with a full set of male and a full set of female parts), but that is not what we are talking about. Humans cannot have a full set of male and female parts; it is not physically possible. People with DSD are not mythological beings. They are real people born with variations in sex anatomy.

Some people born with DSD are gay, some are straight, and some are bisexual. In other words, they have the same range of sexual orientations as people not born with DSD. You cannot predict or determine a person’s sexual orientation by looking at his or her chromosomes, genes, sex glands, or genital anatomy. Humans are much more complicated than that!

People sometimes react negatively to people with DSD because they don’t understand DSD or because DSD challenge their ideas. You can help by educating others. You can also help by not treating a person with a DSD (or his or her family) differently just because he or she has a DSD. If you would like to help financially, you can donate money to an organization that helps families with DSD.