Clinical overview
A newborn with ambiguous genitalia is simultaneously a potential medical emergency, a diagnostic puzzle, and a profoundly sensitive family situation — and the assessment must hold all three at once. The emergency is salt-wasting congenital adrenal hyperplasia (CAH), the commonest cause, which can kill in the first fortnight of life; the puzzle is the structured work-up that establishes the underlying difference (disorder) of sex development (DSD); and the sensitivity is that a family is waiting to be told "is it a boy or a girl?" while you must counsel them honestly that the answer needs time and a team. This objective is to demonstrate the assessment, so the chapter is weighted toward the history, the examination, the investigations and the multidisciplinary process — and toward the communication that must run alongside them.
Two rules anchor the approach. First, do not assign or register a sex hastily — a wrong, irreversible declaration causes lasting harm; sex of rearing is decided by an experienced multidisciplinary team after investigation, with the family. Second, exclude salt-wasting CAH urgently in every case, because it is both the most likely diagnosis and the most lethal if missed. The chapter uses the modern, respectful DSD terminology (which replaced "intersex"/"hermaphrodite") and links to Hyperandrogenism (the shared 21-hydroxylase biology), Informed consent (the consent and communication imperative), and Principles of inheritance.
Core knowledge
Terminology and classification (Chicago Consensus)
Figure E2.1 — The DSD classification (Chicago Consensus), anchored on karyotype: sex-chromosome DSD, 46,XX DSD (androgen excess — commonest is CAH), and 46,XY DSD (under-virilisation).
The 2006 Chicago Consensus (Lawson Wilkins Paediatric Endocrine Society + ESPE) introduced "differences/disorders of sex development (DSD)" — congenital conditions in which development of chromosomal, gonadal or anatomical sex is atypical — and a karyotype-anchored classification (updated by the 2016 Global DSD Update):
- Sex-chromosome DSD — e.g. 45,X (Turner), 47,XXY (Klinefelter), 45,X/46,XY (mixed gonadal dysgenesis), 46,XX/46,XY chimerism.
- 46,XX DSD — usually androgen excess virilising a genetic female; CAH is by far the commonest (see below). Also maternal androgen sources (luteoma, exogenous), aromatase deficiency.
- 46,XY DSD — usually under-virilisation of a genetic male: disorders of testis development (gonadal dysgenesis), of androgen synthesis (5α-reductase deficiency, 17β-HSD), or of androgen action (androgen insensitivity syndrome), and LH/hCG receptor defects.
Congenital adrenal hyperplasia — the one you must not miss
Figure E2.2 — CAH, the one not to miss: the 21-hydroxylase block shunting precursors to androgens (virilised 46,XX, raised 17-OHP) and the salt-wasting neonatal emergency to treat before results.
21-hydroxylase deficiency accounts for the majority of CAH and is the commonest cause of ambiguous genitalia. The enzyme block diverts steroidogenesis away from cortisol (± aldosterone) toward androgens, so:
- A 46,XX infant is virilised (clitoromegaly, labial fusion, a common urogenital sinus) — ambiguous genitalia in a genetic female with normal ovaries and uterus and impalpable gonads.
- 17-hydroxyprogesterone is markedly raised (the diagnostic test).
- The salt-wasting form (aldosterone deficiency) causes an adrenal crisis from about day 3–14 — hyponatraemia, hyperkalaemia, hypovolaemia, hypoglycaemia, shock — which is the lethal emergency. (A 46,XY infant with CAH has normal-appearing male genitalia and presents only with the salt-wasting crisis — easily missed.)
Reading the lesion from the three layers
Sex develops as a chain: the karyotype sets the gonad, the gonad sets the hormones, and the hormones build the ducts and the external genitalia. A DSD is a break somewhere along that chain, and the diagnosis is read backwards from the phenotype to the break. The hormone logic that drives it (anti-Müllerian hormone regresses the Müllerian ducts, testosterone maintains the Wolffian ducts, and dihydrotestosterone virilises the external genitalia) is the embryology of sexual differentiation, and here it becomes a bedside tool.
Three layers are read separately, and the discordance between them points to the lesion:
- The uterus, on ultrasound, reports on AMH. A uterus is present when no functioning testicular tissue made AMH, meaning a 46,XX infant or a dysgenetic 46,XY gonad. A uterus is absent in a 46,XY infant only if AMH worked, which places the lesion downstream, in androgen synthesis or action.
- The internal ducts report on testosterone: the Wolffian structures form only where local testosterone was adequate.
- The external genitalia report on DHT action at the target tissue, not on the testosterone level in the blood.