In this book, we are generally going to use the term “sex” to talk about the physical parts of our bodies that have to do with being male or being female. The aspects of your sex include your “sex chromosomes” (which we will talk more about in a moment), and your sex/reproductive 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 chemical messengers in your body that move through the blood. They help your body develop and function sexually. For example, before you were 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 either 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.) There are pictures of genital development in Figure 5.1 “Genital Development Before Birth”. Genital development is also talked about in more detail there.

We could say that the first stage of sex differentiation happens right at conception. The egg and the sperm each contain 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 could 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 a lot of opportunities for a person to develop along a path that is not the average one for a boy or a girl. 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.”