What is causing my intermittent brown, pink, and red vaginal bleeding for three weeks after passing a gestational sac, with an empty uterus on transvaginal ultrasound, endometrial thickness of 8 mm, quantitative beta‑human chorionic gonadotropin (β‑hCG) of about 6 IU/L, and a negative urine pregnancy test?

Post-Miscarriage Bleeding: Normal Uterine Healing

Your intermittent brown, pink, and red bleeding for three weeks after passing the gestational sac, with an empty uterus on ultrasound, an 8 mm endometrial lining, and near-zero hCG (6 IU/L with negative urine test), represents normal post-miscarriage uterine healing and does not indicate retained products of conception. 1

Why This Bleeding Pattern Is Expected

Your clinical picture is entirely consistent with normal post-miscarriage recovery:

• The 8 mm endometrial thickness is well within normal limits for the post-miscarriage period and does not suggest retained products of conception (RPOC), which would require thickness >20-25 mm to be concerning 1
• Shedding of residual endometrial tissue is a normal component of uterine healing after miscarriage and produces the intermittent spotting you're experiencing 1
• The placental implantation site continues to remodel and heal after tissue expulsion, which can cause intermittent bleeding for several weeks 1
• Ongoing uterine involution involves gradual breakdown and expulsion of the endometrial lining, producing spotting that can persist for several weeks 1

Your hCG Pattern Confirms Resolution

• Your hCG of 6 IU/L with a negative urine pregnancy test indicates near-complete resolution of trophoblastic tissue 2
• Serial hCG measurements should demonstrate progressive decline to undetectable levels (<5 mIU/mL) following complete miscarriage, which your values are approaching 1
• Most qualitative urine pregnancy tests detect hCG at 20-25 mIU/mL, so your negative urine test with serum hCG of 6 confirms the pregnancy has resolved 2

Ultrasound Findings Rule Out Complications

Your two scans showing a clear uterus are reassuring:

• Retained products of conception would show specific ultrasound findings that you do not have: an echogenic endometrial mass with Doppler-detected vascularity, focal endometrial thickening with blood flow, or a discrete identifiable mass 1
• Your 8 mm endometrial stripe is far below the 20-25 mm threshold that would raise concern for RPOC 1
• The absence of a vascular mass on ultrasound essentially excludes significant retained tissue 3, 1

Today's Brief Period-Like Bleeding

The episode of heavier bleeding that stopped after a few hours is consistent with:

• Final expulsion of residual endometrial tissue as your uterus completes its healing process 1
• Normal variation in post-miscarriage bleeding patterns, which can include intermittent heavier episodes 1
• This does not indicate a complication given your reassuring ultrasound findings and declining hCG 1

When to Seek Immediate Evaluation

Return to the emergency department if you develop any of these warning signs:

• Heavy vaginal bleeding (soaking more than one pad per hour for ≥2 consecutive hours) 1
• Fever ≥38°C (100.4°F) or foul-smelling vaginal discharge suggesting endometritis 1
• Hemodynamic instability such as dizziness, syncope, or orthostatic symptoms 1
• Severe abdominal pain that is new or worsening 1

Follow-Up Recommendations

• Repeat quantitative serum hCG in 1-2 weeks to confirm it has declined to undetectable (<5 mIU/mL), which will definitively confirm complete resolution 1
• If your hCG plateaus over 3-4 consecutive weekly measurements or rises >10% across three values within 2 weeks, this would raise concern for gestational trophoblastic disease and require immediate evaluation 1
• Dilation and curettage is NOT indicated in your case, as it should be reserved only for heavy bleeding requiring transfusion, confirmed vascular RPOC on Doppler, or persistent/rising hCG—none of which apply to you 1

Common Pitfall to Avoid

• Do not diagnose retained products of conception solely on endometrial thickness <20 mm; your 8 mm stripe is normal after miscarriage and does not warrant intervention 1
• Intermittent spotting for several weeks is expected and does not indicate a complication when ultrasound and hCG trends are reassuring 1

Your bleeding should gradually taper and resolve completely over the next 1-2 weeks as your hCG reaches zero and your endometrium fully regenerates. The key is that your ultrasounds are clear, your hCG is nearly undetectable, and you have no signs of infection or heavy bleeding.