What tests are used to diagnose and monitor a spontaneous abortion in a patient?

Diagnostic Monitoring for Spontaneous Abortion

Serial quantitative serum β-hCG measurements combined with transvaginal ultrasound are the primary tests used to diagnose and monitor spontaneous abortion. 1

Primary Diagnostic Tests

Serial β-hCG Monitoring

• Obtain quantitative serum β-hCG measurements every 48 hours to assess for appropriate decline or plateau, as failing pregnancies demonstrate levels that fail to rise appropriately or decrease 1, 2
• In viable intrauterine pregnancies, β-hCG typically doubles every 48-72 hours in early pregnancy (53-66% rise over 48 hours), while nonviable pregnancies show declining or plateauing levels 2
• Continue serial measurements until β-hCG falls below 5 mIU/mL to confirm complete resolution and rule out retained products of conception 3
• When β-hCG reaches <1 mIU/mL, this represents complete clearance of all trophoblastic tissue and effectively rules out retained products of conception 3

Transvaginal Ultrasound

• Transvaginal ultrasound is the imaging modality of choice for initial evaluation and should be performed regardless of β-hCG level 1
• Diagnostic criteria for early pregnancy loss include: crown-rump length ≥7 mm without cardiac activity, mean gestational sac diameter ≥25 mm without embryo, or absence of embryo ≥14 days after initial visualization of gestational sac 1
• Ultrasound can differentiate between types of spontaneous abortion: threatened (closed cervix, viable fetus), incomplete (retained tissue visible), complete (empty uterus), or missed abortion (embryonic/fetal demise without expulsion) 1, 4
• An "empty uterus" on ultrasound has 98% reliability for confirming complete spontaneous abortion without need for surgical intervention 4

Diagnostic Algorithm

Initial Assessment

• Obtain baseline quantitative serum β-hCG immediately upon presentation with vaginal bleeding or cramping in early pregnancy 1, 2
• Perform transvaginal ultrasound with Doppler to evaluate for intrauterine gestational sac, embryonic cardiac activity, and assess for retained products 1
• Correlate β-hCG level with ultrasound findings: at β-hCG >3,000 mIU/mL, a gestational sac should be definitively visible on transvaginal ultrasound 5, 2

Follow-Up Monitoring

• Repeat β-hCG in exactly 48 hours to characterize the pregnancy trajectory—this interval is evidence-based for distinguishing viable from nonviable pregnancies 2
• If β-hCG declines appropriately, continue serial measurements weekly until <5 mIU/mL to confirm complete resolution 1, 3
• If β-hCG plateaus (defined as <15% change over 48 hours for two consecutive measurements), further evaluation for retained products or gestational trophoblastic disease is required 2

Verification of Complete Abortion

• Both β-hCG and ultrasound are equally effective for confirming complete abortion, with Kappa agreement coefficient of 0.327, though β-hCG should supplement clinical assessment 6
• Ultrasound showing endometrial thickening >10 mm with Doppler flow suggests retained products of conception and requires intervention 3
• If β-hCG remains elevated or fails to decline to <1 mIU/mL, retained products of conception are likely present 3

Critical Pitfalls to Avoid

• Never use a single β-hCG measurement alone for diagnosis—serial measurements provide meaningful clinical information while isolated values have limited diagnostic utility 2
• Do not defer ultrasound based on "low" β-hCG levels in symptomatic patients, as approximately 22% of ectopic pregnancies occur at levels <1,000 mIU/mL and must be excluded 2
• Avoid waiting for fever to diagnose infection in incomplete or missed abortion—look for maternal tachycardia, purulent cervical discharge, and uterine tenderness as early signs requiring immediate broad-spectrum antibiotics and urgent surgical evacuation 1
• Do not use expectant management for missed abortion due to significantly higher maternal morbidity (60.2% vs 33.0% with active treatment), increased infection risk (38.0% vs 13.0%), and postpartum hemorrhage risk (23.1% vs 11.0%) 1

Essential Preventive Measure

• All Rh-negative women with any type of spontaneous abortion must receive 50 μg anti-D immunoglobulin to prevent alloimmunization, as fetomaternal hemorrhage occurs in 32% of spontaneous abortions 1