Laboratory Tests for Threatened Abortion
For patients presenting with threatened abortion, laboratory tests should include blood type and Rh status, as Rh-negative women require RhIg administration to prevent alloimmunization, especially with heavy bleeding, abdominal pain, or when the event occurs near 12 weeks gestation. 1
Essential Laboratory Tests
- Blood Type and Rh Status: Critical to identify Rh-negative patients who require RhIg prophylaxis to prevent alloimmunization 2, 1
- Complete Blood Count (CBC): To assess for anemia from blood loss and evaluate for signs of infection 3
- Quantitative β-hCG: To confirm pregnancy and establish a baseline for follow-up, though a single low value regardless of gestational age is predictive of abortion 4
- Blood Group Antibody Screen: To identify patients who may already be alloimmunized 1
Additional Tests Based on Clinical Presentation
- Progesterone Level: A single low value is highly predictive of abortion regardless of gestational age 4
- Ultrasound Examination: Though not a laboratory test, it is essential to confirm viability and rule out ectopic pregnancy; absence of fetal life after 9 weeks is invariably followed by abortion 5, 4
- Quantitative Testing for Fetomaternal Hemorrhage: Consider in cases with significant bleeding to determine if additional doses of RhIg are needed for Rh-negative patients 1
Management of Rh-Negative Patients
- RhIg Administration: Administer 50 μg of RhIg within 72 hours to Rh-negative women with threatened abortion, especially with heavy bleeding or abdominal pain 2, 1
- Timing Considerations: RhIg should be given even in early pregnancy since fetal RBCs display RhD antigens from as early as 6 weeks gestation 1
- Dosing: 50 μg is adequate for first trimester losses; if unavailable, use the standard 300 μg dose 1
Clinical Pearls and Pitfalls
- Don't Delay Testing: Prompt laboratory evaluation is crucial as 17-30% of threatened abortions progress to spontaneous abortion 3, 6
- Don't Rely Solely on β-hCG: While important, β-hCG alone cannot accurately predict pregnancy outcome in all cases of threatened abortion 7
- Don't Overlook Rh Status: Failing to identify and treat Rh-negative patients can lead to alloimmunization and serious complications in future pregnancies 1
- Consider Gestational Age: The viable pregnancy rate is highest at 6-8 weeks (61.2%) and lowest at 18-20 weeks (20%) 5
Risk Stratification
- Presence of Subchorionic Hematoma: Associated with higher risk of spontaneous abortion, especially when size exceeds 20 cm² 3
- Maternal Age: Viable pregnancy rates are highest at maternal age 25-29 years (49%) and lowest at 40-44 years (0%) 5
- Hormone Combinations: Combined low values of hCG + estradiol or hCG + estriol are invariably followed by abortion from the sixth and eighth weeks onward, respectively 4