Diagnosis threshold: Endometrial thickness >10 mm with heterogeneous echogenicity AND internal vascularity on Doppler is the most predictive combination. Thin, homogeneous endometrium (<10 mm, no flow) makes RPOC unlikely but does not exclude it — clinical correlation essential.
Sonographic Findings
| Finding | Description | Significance |
|---|---|---|
| Endometrial thickness >10 mm | Single-layer measurement on sagittal TVUS at thickest point, excluding fluid | Most used threshold; sensitivity ~80%, specificity ~65% alone — must combine with echogenicity and Doppler |
| Heterogeneous endometrial echogenicity | Mixed echogenic/hypoechoic material within endometrial cavity; may contain echogenic foci (calcification from prior procedure) | Highly suspicious when combined with vascularity; heterogeneous without flow = likely blood clot |
| Intraendometrial vascularity (color Doppler) | Color flow within endometrial contents; high-velocity, low-resistance waveform on spectral Doppler (RI <0.4 suggests retained trophoblastic tissue) | Most specific finding — presence of flow strongly supports RPOC over hematoma |
| Gestational sac remnant | Recognizable GS, yolk sac, or embryonic tissue within the cavity | Definitive — always represents RPOC |
| Echogenic endometrial mass | Well-defined echogenic intracavitary mass; shadowing if calcified (chronic RPOC) | Chronic RPOC may show calcified foci from prior incomplete evacuation |
| Free fluid in cul-de-sac | Non-specific; may accompany RPOC with hemorrhage or infection | Large free fluid + fever → consider septic RPOC |
Doppler pearls: Always assess the entire myometrium, not just the endometrium. Subinvolution of the placental site (SIPS) shows prominent myometrial vascularity at the implantation site without discrete tissue. Uterine AVM shows turbulent high-velocity low-resistance flow in a myometrial nidus — do NOT curettage an AVM (hemorrhage risk).
Differential Diagnosis
| Differential | Key Distinguishing Features |
|---|---|
| Blood clot / hematoma | No internal Doppler flow; may be echogenic or heterogeneous; changes shape with transducer pressure; resolves on follow-up |
| Uterine AVM | Turbulent mosaic color flow in myometrium; high-velocity low-resistance waveforms; often post-curettage; do NOT curettage — hemorrhage risk |
| Subinvolution of placental site (SIPS) | Myometrial vascularity at implantation site without discrete mass; late postpartum (>6 weeks); low-resistance flow; no tangle of vessels |
| Endometritis | Thickened endometrium with gas (echogenic foci with dirty shadowing); fever, uterine tenderness; may coexist with RPOC |
| Endometrial polyp | Smooth, well-defined echogenic intracavitary lesion; pedicle vessel on Doppler; no recent pregnancy history |
| Gestational trophoblastic disease (GTD) | Markedly elevated β-hCG; heterogeneous "snowstorm" appearance; prominent myometrial vascularity; follows molar pregnancy |
Reporting Checklist — RPOC
- Endometrial thickness (mm) — single layer, at thickest point
- Echogenicity of endometrial contents: homogeneous / heterogeneous / echogenic mass
- Internal vascularity on color Doppler: present / absent
- If flow present: waveform character (high-velocity, low-resistance RI <0.4 = trophoblastic tissue)
- Presence of recognizable gestational sac, yolk sac, or embryo
- Myometrial vascularity: normal / increased (consider AVM or SIPS if prominent)
- Free fluid: none / trace / moderate with location
- Clinical context: post-SAB / post-TOP / post-delivery / post-molar — gestational age at event
- Impression: findings consistent with / not suggestive of RPOC; recommend clinical correlation and β-hCG
References
Kamaya A et al. Retained Products of Conception. Semin Ultrasound CT MR. 2015;36(4):327–35.
Radiopaedia — Retained products of conception