In one line
A disorder of sex development (DSD) is a congenital mismatch between chromosomal, gonadal and phenotypic sex; the neonate with ambiguous genitalia is a social and a medical emergency at once, and the two consultant duties that override everything else are to find and treat salt-wasting congenital adrenal hyperplasia before it kills the baby and to resist assigning a sex, naming, or operating until a multidisciplinary work-up is complete.
The assessment of the ambiguous newborn is built up from first principles at ambiguous genitalia basics; that groundwork — the bedside examination, the parental conversation, the initial panic-management — is assumed here. The focus is the consultant layer: classifying the underlying condition correctly, stratifying the danger, and holding a defensible line through the genuinely contested decisions about surgery, gonadectomy and sex of rearing.
Mechanism & pathophysiology
Sex development runs as a sequence — chromosomal sex sets gonadal sex, gonadal sex sets hormonal output, and hormones build the ducts and the external genitalia — and a DSD is a lesion somewhere along that chain. Reading the chain backwards from the phenotype to the lesion is the whole diagnostic exercise.
Gonadal determination. The bipotential gonad is identical in both sexes until about six weeks. SRY on the short arm of the Y chromosome is the master switch: it upregulates SOX9, which drives the supporting cells down the Sertoli (testis) pathway. Without a functioning SRY/SOX9 cascade — and with the pro-ovarian signals WNT4/RSPO1 and FOXL2 unopposed — the gonad becomes an ovary. The cascade is dose- and timing-sensitive: a mutation in SRY, SOX9, NR5A1 (SF1), WT1, MAP3K1 or the dosage-sensitive NR0B1 (DAX1) locus can produce a 46,XY individual with dysgenetic or frankly ovarian gonads, and translocation of SRY onto an X can produce a 46,XX testicular DSD.
Duct and external-genital differentiation are hormone-driven, and the two hormones act independently. Once a testis forms, Sertoli cells secrete anti-Müllerian hormone (AMH), which regresses the Müllerian ducts (otherwise the uterus, tubes and upper vagina). Leydig cells secrete testosterone, which stabilises the Wolffian ducts into epididymis, vas and seminal vesicles. The external genitalia are a separate matter again: they masculinise only under dihydrotestosterone (DHT), the 5α-reductase product of testosterone, acting through the androgen receptor. This separation is the key to several conditions. A 46,XY fetus with 5α-reductase type 2 deficiency makes testosterone (so the internal Wolffian structures form and AMH regresses the uterus) but cannot make DHT, so the external genitalia are undervirilised at birth and then virilise at puberty under the testosterone surge — the classic "born a girl, becomes a boy" history. A 46,XY fetus with complete androgen insensitivity (CAIS) makes testosterone and AMH normally but cannot respond to either androgen: no uterus (AMH worked), no Wolffian structures, female external genitalia, a blind-ending vagina and intra-abdominal testes — phenotypically female, 46,XY, presenting later with primary amenorrhoea or inguinal "hernias" that are testes.
The 2006 Chicago consensus classification is the framework every answer is organised around. It abandoned the stigmatising "intersex/pseudohermaphrodite" language for three karyotype-anchored groups:
- 46,XX DSD — the gonad is an ovary, the karyotype is XX, and the genitalia are virilised. The overwhelmingly commonest cause, and the one that can kill, is congenital adrenal hyperplasia (CAH), almost always 21-hydroxylase deficiency: the adrenal cannot make cortisol (and, in the salt-wasting form, aldosterone), ACTH drives adrenal hyperplasia, and the blocked precursors are shunted into the androgen pathway, virilising a genetically female fetus in utero. Rarer 46,XX causes include aromatase deficiency and maternal androgen exposure (luteoma, exogenous androgens).
- 46,XY DSD — a Y chromosome and a testis (or dysgenetic gonad) but undervirilised genitalia. Causes split into disorders of androgen action/synthesis (CAIS and partial AIS, 5α-reductase deficiency, testosterone-biosynthesis defects) and disorders of gonadal development (complete or partial gonadal dysgenesis, e.g. Swyer syndrome — a 46,XY phenotypic female with streak gonads and a uterus, because dysgenetic gonads make neither AMH nor testosterone).
- Sex-chromosome DSD — the abnormality is in the chromosome complement itself: 45,X (Turner syndrome), 47,XXY (Klinefelter syndrome), 45,X/46,XY mixed gonadal dysgenesis (asymmetric gonads — a testis on one side, a streak on the other, often a uterus, frequently ambiguous genitalia and a high tumour risk), and the chimeric 46,XX/46,XY ovotesticular DSD (both ovarian and testicular tissue in the same individual, in one gonad as an ovotestis or in opposite gonads; the modern term for the old "true hermaphroditism"). The common 46,XX ovotesticular DSD, which predominates in South African series, is classified under 46,XX DSD rather than here, because its karyotype is a single XX cell line.
Holding this classification is not an academic exercise: it tells you which neonate is at risk of an adrenal crisis (46,XX CAH), which has a cancer-prone intra-abdominal gonad (dysgenetic or Y-bearing), and which will declare a gender trajectory that may diverge from the natal genital appearance.
The mechanistic detail that earns the diagnosis is the discordance between the three layers — Müllerian structures, Wolffian structures, and the external phenotype — because each is driven by a different signal and so points to a different lesion. A uterus that is present means AMH was either never made (no functioning Sertoli tissue: a 46,XX, or a dysgenetic 46,XY gonad) or was made but not the relevant problem; a uterus that is absent in a 46,XY individual confirms AMH worked, so the lesion is downstream in androgen synthesis or action (AIS, 5α-reductase deficiency, a testosterone-synthesis block). Virilised external genitalia in a 46,XX infant with a normal uterus and ovaries localises the androgen source to the adrenal (CAH) or, rarely, a maternal/placental source — not to the gonad. This three-layer logic, applied to the ultrasound and the steroid profile, is what converts a karyotype plus a hormone panel into a specific diagnosis rather than a list of possibilities.
Assessment
A newborn whose genitalia cannot be confidently called male or female is never assigned a sex on the labour ward, given a gendered name, or sent home before the work-up runs. The two parallel imperatives — exclude life-threatening salt-wasting CAH, and establish the underlying diagnosis without prejudging the answer — drive everything.
The features that make genitalia genuinely ambiguous (and mandate the full pathway, not reassurance): an apparent male with non-palpable testes, hypospadias with a bifid scrotum, or a stretched phallus length below the normal range; an apparent female with clitoromegaly, posterior labial fusion or a palpable gonad in the labioscrotal fold or groin; any discordance between an antenatal karyotype and the genital appearance; and a family history of DSD, neonatal death or consanguinity.
The salt-wasting CAH crisis is the immediate danger and the assessment is built to catch it. A virilised 46,XX infant with CAH looks like the "safe" end of ambiguity — a girl who is merely virilised — yet she is the one who can collapse. The aldosterone deficiency causes salt wasting that typically declares itself at day 7–14 of life as vomiting, poor feeding, lethargy, weight loss, dehydration and shock, with the biochemical signature of hyponatraemia, hyperkalaemia and metabolic acidosis (and hypoglycaemia from cortisol deficiency). In a setting without universal newborn CAH screening — which is the South African public-sector reality — there is no biochemical safety net, so the crisis is the presentation, and a missed or late diagnosis carries real neonatal mortality. The practical corollary: any sick, dehydrated, hyperkalaemic neonate, and any ambiguous neonate, gets a serum 17-hydroxyprogesterone and an electrolyte panel urgently, and you do not wait for it to be abnormal before treating a shocked baby empirically.
The structured first-line work-up (run in parallel, not in series):