Transgender and Gender-Affirming Science Prerequisites
Start from one idea, because everything else in this chapter hangs off it: the human body is not one switch but a stack of separate developmental and endocrine decisions, and they do not have to agree. Chromosomes, gonads, internal ducts, external genitalia, the hormone environment a body actually sits in, a person's internal sense of gender, their sexual function and their fertility are different layers. In the common case they line up so neatly that we forget they are separable — but in disorders/differences of sex development (DSD), and in transgender and gender-diverse care, they can come apart. A clinician who reasons from one layer to another ("she's a woman, so she has a uterus and no prostate") will miss disease.
So the science prerequisite for safe O&G care is simply this: learn to read each layer on its own, then put them back together for the specific clinical question in front of you. Everything below — the embryology, the steroid biology, the screening logic — is just the detail that lets you do that without making an unsafe assumption.
In practice that means O&G touches every kind of body and goal. A transgender man may need contraception, cervical screening, pregnancy care, fertility preservation or treatment for pelvic pain. A transgender woman may need endocrine monitoring, breast health advice, fertility counselling, VTE risk assessment or postoperative genital care. A non-binary patient may need any combination of these.
The single core rule that falls out of the layered model is:
screen, counsel and treat according to organs present, hormone exposure, risk factors and the patient's goals — never according to a recorded gender, an assumption or an appearance.
This chapter is not a full gender-affirming care protocol, and it is deliberately not a place to memorise doses; the prescribing detail belongs to the Intermediate and Final courses. It is the basic-science foundation that prevents unsafe assumptions and lets you understand why a guideline says what it says.
Why the Layers Can Differ: the Developmental Cascade
Before the clinical layers make sense, you need the mechanism that builds them — because the same cascade that produces typical anatomy is what allows the layers to come apart, both in DSD and (later, hormonally) in gender-affirming care.
Early embryos are bipotential. At around five to seven weeks the gonad is an "indifferent" ridge that could become either a testis or an ovary, and both duct systems are laid down side by side: the paramesonephric (Müllerian) ducts (which can become tubes, uterus, cervix and upper vagina) and the mesonephric (Wolffian) ducts (which can become epididymis, vas deferens and seminal vesicles). External genitalia are likewise identical in the two sexes until about ten weeks — one genital tubercle, paired genital folds and paired labioscrotal swellings.
What tips the cascade one way or the other is a short sequence of molecular switches:
| Step | Switch | Consequence if present | Consequence if absent |
|---|---|---|---|
| Chromosomal | the SRY gene (normally on the Y) | drives the indifferent gonad toward a testis | the gonad follows the ovarian path |
| Gonadal | Sertoli cells form; Leydig cells form | Sertoli cells secrete anti-Müllerian hormone (AMH); Leydig cells secrete testosterone | no AMH, low testosterone |
| Ductal | AMH + testosterone act locally | AMH regresses the Müllerian ducts; testosterone maintains the Wolffian ducts | Müllerian ducts persist (tubes/uterus/cervix); Wolffian ducts regress |
| External genital | dihydrotestosterone (DHT) via 5α-reductase | tubercle → penis, folds fuse → male urethra, swellings fuse → scrotum | tubercle → clitoris, folds → labia minora, swellings → labia majora |
Three things in that table do the heavy lifting for the rest of the chapter. First, the gonad decides the ducts, not the chromosomes directly — it is the hormone signals (AMH, testosterone, DHT) that build the anatomy, which is exactly why hormone exposure later in life is so powerful. Second, internal ducts and external genitalia are built by partly different signals (AMH and testosterone for ducts; DHT for external) — so they can diverge: a gonad, a duct system and an external appearance need not "match." Third, DHT is testosterone amplified by an enzyme (5α-reductase) only in selected tissues — the same enzyme that masculinises genital skin in the embryo is the one that drives clitoral growth, genital-skin change and hair-follicle effects on testosterone therapy decades later.
This is the engine behind DSD (e.g. a 46,XY person with complete androgen insensitivity develops along a female external path despite testes and testosterone, because the androgen receptor cannot read the signal) and the reason the layers in the next section are genuinely independent variables rather than a single fact about a person.
Sex Development Layers
With the cascade in mind, the layers below are no longer arbitrary categories — each is the output of one step in that developmental sequence, plus the hormone environment the body sits in now. They may align in common patterns or differ in DSD and gender-diverse care.
| Layer | Basic science question | O&G relevance |
|---|---|---|
| Chromosomal sex | XX, XY or variation? | DSD genetics, gonadal risk, fertility potential |
| Gonadal sex | Ovaries, testes, dysgenetic gonads, ovotestis? | Hormone production, gametes, malignancy risk |
| Internal ducts | Mullerian and/or Wolffian structures present? | Uterus, cervix, tubes, prostate, vas/epididymis |
| External genital anatomy | Vulva/vagina/clitoris, penis/scrotum, variations, surgery? | Examination, sexual function, urinary and surgical care |
| Endocrine milieu | Oestrogen, testosterone, progestin, blockers, endogenous gonads? | Bone, bleeding, breast, VTE, fertility, metabolism |
| Gender identity | Person's internal sense of gender | Communication, consent, dysphoria-sensitive care |
| Reproductive goals | Pregnancy, contraception, fertility preservation, no fertility goal? | Counselling and method choice |
The unsafe shortcut is to infer one layer from another. A masculine-presenting patient may have a cervix and need cervical screening. A transfeminine patient may have a prostate and need prostate-aware care. A person on testosterone may still ovulate and conceive.
Steroid Receptor and Axis Basics
Gender-affirming hormones are not exotic drugs; they are the same sex steroids the body already makes, given to shift the hormone environment. So they work through ordinary endocrine physiology, and the first principle is the steroid-receptor mechanism you met in endocrine physiology.
Sex steroids are all built from cholesterol on a shared synthetic pathway, distinguished mainly by carbon count: progestogens have 21 carbons, androgens (testosterone, DHT) have 19, and oestrogens have 18. The conversion of a 19-carbon androgen to an 18-carbon oestrogen is done by the enzyme aromatase — a fact that matters more than it looks, because it means a meaningful share of a person's oestrogen can come from peripheral conversion of androgens in fat and other tissues, not only from a gonad.
Because they are small lipophilic molecules, sex steroids do something water-soluble (peptide) hormones cannot: they cross the plasma membrane directly. The mechanism then runs:
- In blood, most of the hormone travels bound to carrier proteins, chiefly sex hormone–binding globulin (SHBG); only the small free fraction is biologically active. (Testosterone is roughly 97% protein-bound.)
- The free hormone diffuses into the cell and binds an intracellular receptor. Androgen, oestrogen and progesterone receptors are part of the nuclear-receptor superfamily and behave as ligand-activated transcription factors.
- Binding triggers a conformational change: chaperone (heat-shock) proteins dissociate, the receptor dimerises and translocates to the nucleus, where it binds specific hormone-response elements on DNA.
- There it switches gene transcription up or down, depending on whether co-activator or co-repressor proteins are recruited. This is why the same receptor produces different effects in different tissues — the co-regulator mix is cell-specific.