Boys and girls are different. It seems so obvious. Yet some medical conditions may cause some of those differences to become confused. And then telling apart boys from girls can prove challenging.
It’s one measure of how complex human biology is.
When it comes to whether someone looks like a boy or girl, hormones clearly run the show. For instance, a newborn girl’s genitals may appear somewhat or totally male if that baby had encountered too much of the hormone testosterone (tess-TOSS-tur-own) in the womb. Similarly, too little of this hormone can impair the development of a boy’s reproductive organs.
But male hormones shape other organ systems as well. These include the kidneys and bladder — but most importantly the brain. At birth and throughout life, for instance, the size and function of certain regions in the brain will differ between males and females.
Testosterone is an androgen, or male sex hormones. So how can it end up in the womb of a woman? She might have become exposed during pregnancy to a medicine containing this hormone. More commonly, genetic changes — called mutations — will tell her fetus to produce too much testosterone or to make this hormone at the wrong time. (Both males and females make the hormone, but in very different amounts). This could trigger a small but critical change in how a girl’s body develops.
When this happens very early in development, a baby may be born with one of several conditions. Collectively, they’re known as differences or disorders of sex development, or DSDs. (There is no scientific evidence showing that DSDs cause or are linked to transgender identity.)
DSDs are rare, notes William Reiner. He is a child and adolescent psychiatrist at the University of Oklahoma Health Sciences Center in Oklahoma City. He is also a pediatric urologist, specializing in diseases affecting the urinary tract and conditions affecting male reproductive organs.
The best studied DSD is something known as congenital adrenal hyperplasia, or CAH. The grape-size adrenal glands manufacture small amounts of testosterone — in everyone. A mutation in genes might instruct these glands to produce an oversupply of androgens. This mutation would not likely affect boys. They already make lots of androgens, so their bodies would hardly notice a little more.
Girls born with CAH, however, can appear masculinized — more boy-like. In some cases, their reproductive anatomy might slightly, or even strongly, resemble a boy’s. Doctors refer to this condition as intersex.
In severe cases, a baby that is genetically a girl may at birth visually appear to be a boy. Babies born with characteristics of both sexes sometimes undergo surgery soon after birth. This would make their genitals look more characteristic of their genetic sex. Other times, doctors and parents together must decide which gender to assign a baby.
Reiner often sees patients who are born with DSDs and have intersex features. He also studies children and teens who transition to a different gender (the opposite of the one they had been assigned at birth, based on their apparent biological sex). Some of these children are transgender. Others may have encountered conditions in the womb that altered how parts of their body (such as the genitals) developed.
Another type of genetic error, or mutation, prevents the body from making an enzyme needed to produce DHT. This hormone is more powerful than testosterone in differentiating the male body. So too little of this enzyme could cause the bodies of male children to appear feminized. That means their genitals may somewhat — or even totally — resemble a girl’s.
What does this all mean? Says Reiner, “You cannot necessarily tell by looking at the genitals whether you’re going to have a child who has a male or a female gender identity.”
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adolescent Having to do with the transitional stage of physical and psychological development that begins at the onset of puberty, typically between the ages of 11 and 13, and ends with adulthood.
adrenal gland Hormone-producing glands that sit at the top of the kidneys.
androgen A family of powerful male sex hormones.
congenital A term that refers to conditions that are present from birth, either because they were inherited or occurred as a fetus developed in the womb.
congenital adrenal hyperplasia A genetic disorder that causes the adrenal glands to make too much testosterone. This could create developmental changes in the womb that cause baby girls to be born with features that made them appear partly or totally male.
development (in biology) The growth of an organism from conception through adulthood, often undergoing changes in chemistry, size and sometimes even shape.
dihydrotestosterone (DHT) A male sex hormone, or androgen, that plays an important role in the development of male physical characteristics and reproductive anatomy.
enzymes Molecules made by living things to speed up chemical reactions.
feminize (in biology) For a male person or animal to take on physical, behavioral or physiological traits considered typical of females.
fetus (Adj. fetal) The term for a mammal during its later-stages of development in the womb. For humans, this term is usually applied after the eighth week of development.
gene (adj. genetic) A segment of DNA that codes, or holds instructions, for producing a protein. Offspring inherit genes from their parents. Genes influence how an organism looks and behaves.
genitals or genitalia The visible sex organs.
gonads The reproductive organs that make eggs (in females) and sperm (in males).
hormone (in zoology and medicine) A chemical produced in a gland and then carried in the bloodstream to another part of the body. Hormones control many important body activities, such as growth. Hormones act by triggering or regulating chemical reactions in the body.
intersex Animals or humans that display characteristics of both male and female reproductive anatomy.
masculinize (in biology) For a female person or animal to take on physical, behavioral or physiological traits considered typical of males.
mutation Some change that occurs to a gene in an organism’s DNA. Some mutations occur naturally. Others can be triggered by outside factors, such as pollution, radiation, medicines or something in the diet. A gene with this change is referred to as a mutant.
ovary (plural: ovaries) The organ in the females of many species that produce eggs.
pediatrics A field of medicine that has to do with children and especially child health.
psychiatry A field of medicine where doctors study and treat diseases of the human mind. Treatments may consist of talking therapies, prescription drugs or both. People who work in this field are known as psychiatrists.
sex A person’s biological status, typically male or female. There are a number of indicators of biological sex, including sex chromosomes, gonads, internal reproductive organs, and external genitals.
testis (plural: testes) The organ in the males of many species that makes sperm, the reproductive cells that fertilize eggs. This organ also is the primary site that makes testosterone, the primary male sex hormone.
testosterone Although known as male sex hormone, females make this reproductive hormone as well (generally in smaller quantities). It gets its name from a combination of testis (the primary organ that makes it in males) and sterol, a term for some hormones. High concentrations of this hormone contribute to the greater size, musculature and aggressiveness typical of the males in many species (including humans).
transgender Someone who has a gender identity that does not match the sex they were assigned at birth.
urology The medical field that deals with diseases of the male and female urinary tract and conditions affecting male reproductive organs. Doctors who specialize in this area are known as urologists.
womb Another name for the uterus, the organ in which a fetus grows and matures in preparation for birth.