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Ambiguous genitalia in the newborn

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Endocrine & Reproductive Presentations/Sex development

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A newborn whose external genitalia are not unambiguously male or female: an enlarged clitoris with labial fusion, an underdeveloped phallus with hypospadias and undescended testes, asymmetric gonadal position, or any combination that prevents confident sex assignment at birth. The clinical posture is two-fold and simultaneous: rule out life-threatening salt-wasting CAH today (the only diagnosis in this differential that can kill in the first two weeks), and build the multidisciplinary team (endocrinology, urology, genetics, mental health, neonatology) before any sex assignment, surgical decision, or birth-certificate filing.

Three foundational data points organize the work-up:

  1. Karyotype (or rapid X/Y FISH on interphase nuclei from buccal smear or peripheral blood): the first sorting axis is 46,XX vs 46,XY vs sex-chromosome DSD.
  2. Gonad position and Müllerian structures on pelvic and inguinal ultrasound: palpable gonads in labioscrotal folds are usually testes; impalpable gonads with a uterus point toward 46,XX or sex-chromosome DSD.
  3. Hormone profile (17-hydroxyprogesterone, testosterone, DHT, AMH, electrolytes, cortisol, ACTH, LH, FSH) drawn in the first 24-72 hours.

The diagnostic flow then branches by karyotype.

46,XX DSD (virilized female)

  • Congenital adrenal hyperplasia (CAH): the single most important diagnosis in the differential because the salt-wasting form is life-threatening.
    • 21-hydroxylase deficiency (CYP21A2, AR): ~95% of CAH. Elevated 17-OHP on newborn screening or directly assayed. Salt-wasting form presents day 7-14 with hyponatremia, hyperkalemia, hypoglycemia, and shock.
    • 11β-hydroxylase deficiency (CYP11B1): virilization + hypertension (excess deoxycorticosterone) rather than salt wasting.
    • 3β-hydroxysteroid dehydrogenase deficiency (HSD3B2): both 46,XX virilization and 46,XY undervirilization possible.
  • Maternal androgen exposure: maternal androgen-secreting tumor, exogenous androgens, untreated maternal CAH, aromatase deficiency in the mother.
  • Aromatase deficiency (CYP19A1, AR; fetal aromatase deficiency): inability to convert androgens to estrogens in placenta and fetus; virilization of the 46,XX fetus and the mother during pregnancy.
  • Ovotesticular DSD: rare; both ovarian and testicular tissue present.
  • Placental aromatase deficiency and other rare causes.

46,XY DSD (undervirilized male)

  • Disorders of androgen synthesis:
    • 5α-reductase deficiency (SRD5A2, AR): inability to convert testosterone to DHT. External genitalia female or ambiguous at birth; virilization at puberty as testosterone rises. T:DHT ratio > 10 is the screen.
    • 17β-hydroxysteroid dehydrogenase 3 deficiency (HSD17B3): similar phenotype.
    • Cytochrome P450 oxidoreductase deficiency (POR): combined 17α-hydroxylase and 21-hydroxylase block; can have skeletal anomalies (Antley-Bixler-like).
    • 3β-HSD2 (HSD3B2): undervirilization in 46,XY plus virilization in 46,XX siblings.
  • Disorders of androgen action:
    • Androgen insensitivity syndrome (AR, X-linked):
      • Complete AIS: female external phenotype, blind-ending vagina, no Müllerian structures (AMH from testes regresses them), testes in inguinal canals or labia.
      • Partial AIS: ambiguous genitalia at birth.
  • Disorders of testis development (gonadal dysgenesis):
    • Complete (Swyer syndrome): 46,XY with streak gonads, female internal and external phenotype, normal Müllerian structures (no AMH because no functional testis). Genes: SRY, NR5A1, MAP3K1, DHH, DHX37, WT1, others.
    • Partial gonadal dysgenesis: ambiguous genitalia.
  • Leydig cell hypoplasia (LHCGR): undervirilization due to LH-receptor defect.
  • Persistent Müllerian duct syndrome (AMH, AMHR2, AR): 46,XY phenotypic male with persistent uterus and fallopian tubes; often discovered at inguinal hernia repair.
  • Smith-Lemli-Opitz syndrome (DHCR7, AR): 46,XY undervirilization + 2-3 toe syndactyly + microcephaly + dysmorphism; cholesterol biosynthesis defect.

Sex-chromosome DSD

  • Mixed gonadal dysgenesis (45,X / 46,XY mosaic): asymmetric gonads (one streak, one testis), ambiguous genitalia, somatic Turner-like features in some.
  • Ovotesticular DSD with sex-chromosome variants.
  • 45,X / 46,XX mosaicism, 47,XXY variants.
  • Ambiguous genitalia + electrolyte abnormalities or impending shock in week 1-2 → salt-wasting CAH. Do not wait for the karyotype; treat empirically with hydrocortisone and fludrocortisone, draw labs first.
  • Bilateral non-palpable gonads in an apparently female newborn → a virilized 46,XX (often CAH) until proven otherwise.
  • Apparently female newborn with palpable inguinal masses → 46,XY DSD with intra-canalicular testes; consider complete AIS, gonadal dysgenesis variants.
  • Apparently female newborn or child with virilization at puberty → 5α-reductase deficiency or 17β-HSD3 deficiency.
  • 46,XY with hypospadias + 2-3 toe syndactyly + microcephaly → Smith-Lemli-Opitz; send 7-dehydrocholesterol.
  • Asymmetric gonads on imaging or exam → mixed gonadal dysgenesis (45,X/46,XY mosaic); karyotype on peripheral blood may miss it, consider FISH on gonadal tissue.

Drawn within the first 24-72 hours and run urgently:

  1. Karyotype (peripheral blood) and rapid SRY / Y-chromosome FISH (interphase nuclei, hours not days). Some centers add chromosomal microarray in parallel.
  2. 17-hydroxyprogesterone (CAH screen; check newborn screen result if available, repeat serum level).
  3. Basic electrolytes, glucose, ACTH, cortisol drawn serially (the salt-wasting clinical signs emerge over days; track sodium, potassium, weight).
  4. Testosterone, DHT, androstenedione; calculate T:DHT ratio (> 10 supports 5α-reductase deficiency once the postnatal HPG surge begins, around day 14 through ~3 months).
  5. AMH (Sertoli function; present in functional testis, absent in gonadal dysgenesis and 46,XX).
  6. LH, FSH (gonadotropins).
  7. Pelvic and inguinal ultrasound for uterus, ovaries, gonads, adrenal hyperplasia.
  8. Urine steroid profile (gas chromatography / mass spectrometry) if a steroidogenic block beyond 21-hydroxylase is suspected; this picks up rarer CAH variants.
  9. Targeted gene panel (DSD panel covering AR, SRD5A2, HSD17B3, NR5A1, SRY, MAP3K1, DHH, CYP21A2, CYP11B1, HSD3B2, POR, LHCGR, AMH, AMHR2, WT1, DHCR7, STAR, others) once the karyotype is back.
  10. 7-dehydrocholesterol if Smith-Lemli-Opitz features are present.
  • Salt-wasting CAH is the only diagnosis here that can kill the patient in the first two weeks. Track sodium, potassium, weight, and 17-OHP every day until you have an answer; treat empirically if shock develops.
  • Sex assignment is a team decision after diagnostic clarity, made with the family. Do not assign on the delivery floor. "Your baby's body is healthy and we are figuring out the details" is the right initial framing.
  • Karyotype + AMH + T:DHT ratio + ultrasound is the diagnostic core. Hormone levels need timing-appropriate interpretation (postnatal HPG surge, then quiescence in childhood).
  • Asymmetric gonads on imaging is mixed gonadal dysgenesis (45,X/46,XY mosaic) until proven otherwise. Tumor risk in dysgenetic gonads with Y material is significant; gonadectomy timing is decided in concert with urology and endocrinology.
  • The genetic counselor's role is large here. Family education, recurrence risk, gonadal cancer risk, fertility implications, sex-of-rearing discussions, and connecting families to peer support are all part of the work-up, not afterthoughts.