How do incomplete, missed, and threatened abortions differ in clinical presentation, ultrasound findings, and management?

Distinguishing Incomplete, Missed, and Threatened Abortion

Clinical Presentation

Threatened Abortion

Threatened abortion presents with vaginal bleeding and/or cramping while the cervix remains closed and the fetus is still viable. 1, 2

• Bleeding ranges from light spotting to moderate flow 1
• Cramping is typically mild to moderate, lower abdominal 1
• Cervical os is closed on speculum examination 1, 3
• Uterine size is appropriate for gestational age 3
• Patient is hemodynamically stable 2

Incomplete Abortion

Incomplete abortion is characterized by partial expulsion of products of conception with an open cervical os and ongoing bleeding. 1

• Moderate to heavy vaginal bleeding, often with clots 1
• Moderate to severe cramping pain 1
• Cervical os is open, with tissue visible in the cervical canal or vagina 1
• Patient may report passage of tissue 1
• Risk of hemorrhage requiring transfusion if hemoglobin drops below 7 g/dL 1

Missed Abortion (Embryonic/Fetal Demise)

Missed abortion involves confirmed embryonic or fetal death without spontaneous expulsion of conception products. 1, 2

• Minimal or absent vaginal bleeding 1, 4
• Minimal or no cramping 1
• Cervical os is closed 1
• Loss of pregnancy symptoms (breast tenderness, nausea) 5
• Uterine size may be smaller than expected for dates 3

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Ultrasound Findings

Threatened Abortion

• Intrauterine gestational sac with yolk sac or embryo present 2
• Cardiac activity is visible (most critical finding) 2
• Appropriate crown-rump length for gestational age 2
• May show subchorionic hemorrhage 5
• No cervical dilation on transvaginal imaging 1

Incomplete Abortion

• Heterogeneous echogenic material within the endometrial cavity (retained products of conception) 1
• Endometrial thickness typically >15 mm 5
• Gestational sac may be disrupted or absent 5
• No visible embryo with cardiac activity 1
• Cervical canal may contain tissue 1

Missed Abortion

Definitive ultrasound criteria establish the diagnosis without requiring prolonged observation: 2

• Crown-rump length ≥7 mm without cardiac activity (diagnostic) 1, 2
• Mean sac diameter ≥25 mm without visible embryo (diagnostic) 1, 2
• Absence of embryo ≥14 days after initial gestational sac visualization (diagnostic) 2
• Absence of embryo with cardiac activity ≥11 days after visualization of gestational sac with yolk sac 2
• Irregular or collapsed gestational sac 1

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β-hCG Patterns

Threatened Abortion

Serial β-hCG measurements 48 hours apart show appropriate rise (53-66% increase) in viable pregnancies. 2

• β-hCG rises appropriately if pregnancy remains viable 2
• Plateau (change <15% over 48 hours) suggests abnormal pregnancy 2
• Rise of 10-53% over 48 hours raises suspicion for ectopic or non-viable pregnancy 2
• Single β-hCG measurements have limited diagnostic value 6, 2

Incomplete Abortion

• β-hCG levels are typically elevated but fail to rise appropriately or decline 6
• Serial measurements show declining trend 6
• Levels may remain detectable for several weeks after tissue passage 6

Missed Abortion

• β-hCG levels fail to rise appropriately or plateau 6, 2
• Levels eventually decline but may remain elevated for weeks 6
• Plateau over 4 consecutive values spanning 3 weeks may indicate gestational trophoblastic neoplasia 2

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Management Approach

Threatened Abortion

Obtain transvaginal ultrasound immediately regardless of β-hCG level to confirm viability and exclude ectopic pregnancy. 2

• If cardiac activity is present at 6-7 weeks, this is the most powerful positive prognostic factor 2
• Serial β-hCG monitoring every 48 hours if ultrasound is indeterminate 2
• Repeat ultrasound in 7-10 days if gestational sac <25 mm without embryo 6, 2
• All Rh-negative women receive 50 μg anti-D immunoglobulin 1, 2
• Pelvic rest and activity modification (though evidence is limited) 7
• Return immediately for severe pain, heavy bleeding, or hemodynamic instability 2

Incomplete Abortion

Surgical evacuation via vacuum aspiration is the gold standard for moderate-to-severe bleeding, with the lowest complication rates. 1

• Vacuum aspiration: hemorrhage 9.1%, infection 1.3%, retained tissue 1.3% 1
• Medical management with misoprostol 600-800 μg vaginally has 91.5% success rate but higher bleeding (28.3%) and infection (23.9%) risks 1
• Expectant management is contraindicated with profuse bleeding 1
• Urgent laboratory studies: CBC, coagulation parameters, type and cross-match 1
• Transfuse if hemoglobin <7 g/dL or ongoing hemorrhage 1
• All Rh-negative women receive 50 μg anti-D immunoglobulin 1

Missed Abortion

Active evacuation is required; expectant management is absolutely contraindicated due to risks of infection, coagulopathy, and maternal sepsis. 1, 2

• For gestational age <9 weeks: Medical management with mifepristone 200 mg + misoprostol 800 μg (80% success rate) or surgical evacuation 1
• For gestational age 9-12 weeks: Surgical evacuation preferred 1
• For gestational age >12 weeks: Dilation and evacuation (D&E) is the procedure of choice 1
• If infection is suspected (maternal tachycardia, purulent discharge, uterine tenderness), initiate broad-spectrum antibiotics immediately and proceed with urgent evacuation 1, 2
• Do not wait for fever to diagnose infection 1
• All Rh-negative women receive 50 μg anti-D immunoglobulin 1, 2

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Critical Diagnostic Pitfalls

Over-Reliance on β-hCG Discriminatory Thresholds

The traditional discriminatory threshold of 3,000 mIU/mL has virtually no diagnostic utility (positive likelihood ratio 0.8, negative likelihood ratio 1.1) and should not be used to exclude ectopic pregnancy or delay imaging. 6, 2

• Approximately 22% of ectopic pregnancies occur at β-hCG <1,000 mIU/mL 6, 2
• Never defer ultrasound based on "low" β-hCG levels in symptomatic patients 2
• Do not use β-hCG value alone to exclude ectopic pregnancy (Level B recommendation, ACEP) 6, 2

Premature Diagnosis of Pregnancy Loss

Do not diagnose missed abortion based solely on absence of yolk sac or embryo unless mean sac diameter is ≥25 mm. 2

• Absence of cardiac activity in embryo <7 mm CRL requires follow-up imaging 2
• When gestational sac is present but no embryo is seen and MSD <25 mm, schedule repeat ultrasound in 7-10 days 6, 2

Misdiagnosis of Incomplete vs. Ectopic Pregnancy

Careful ultrasound evaluation is essential to differentiate incomplete abortion from ectopic pregnancy, as both can present with bleeding and open cervix. 1

• Presence of intrauterine gestational sac essentially excludes ectopic pregnancy in spontaneous conceptions 2
• Extra-ovarian adnexal mass without intrauterine pregnancy has positive likelihood ratio of 111 for ectopic pregnancy 2