## Ultrasound Diagnosis of Ectopic Pregnancy **Key Point:** The **absence of an intrauterine gestational sac (IUP) in the presence of a positive β-hCG** is the earliest and most reliable sign of ectopic pregnancy on transvaginal ultrasound. ### Timeline of Ultrasound Findings in Normal Pregnancy | β-hCG Level | Expected Ultrasound Finding | Timing | |-------------|----------------------------|--------| | 1,000–2,000 mIU/mL | Gestational sac in uterus (2–3 mm) | 4.5–5 weeks | | 5,000–6,000 mIU/mL | Yolk sac visible | 5–5.5 weeks | | 10,000–20,000 mIU/mL | Fetal pole with cardiac activity | 5.5–6 weeks | | >20,000 mIU/mL | Clear fetal heart rate (>100 bpm) | 6+ weeks | **High-Yield:** The **discriminatory zone** (β-hCG level above which an IUP should be visible) is approximately 1,500–2,500 mIU/mL on transvaginal ultrasound. If β-hCG exceeds this threshold and no IUP is seen, ectopic pregnancy must be excluded. **Clinical Pearl:** Absence of IUP with positive β-hCG does not prove ectopic pregnancy — it may indicate: - Very early intrauterine pregnancy (<4.5 weeks) - Heterotopic pregnancy (simultaneous intrauterine and ectopic) - Miscarriage - Molar pregnancy Serial β-hCG measurement (48-hour doubling) and repeat ultrasound are essential to differentiate these. **Mnemonic:** **DISC** — Discriminatory zone, IUP absent, Serial β-hCG needed, Confirm with repeat Scan.
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