Management of Threatened Abortion
For threatened abortion in the first trimester, perform pelvic ultrasound regardless of β-hCG level to rule out ectopic pregnancy, administer vaginal micronized progesterone 400 mg twice daily if there is a history of prior pregnancy losses, and give 50 µg anti-D immunoglobulin to Rh-negative women with documented pregnancy loss. 1, 2
Immediate Diagnostic Evaluation
Ultrasound Assessment - Critical First Step
Perform transvaginal ultrasound in all clinically stable patients with threatened abortion, regardless of β-hCG level. 1 The common practice of deferring ultrasound when β-hCG is below a discriminatory threshold is dangerous—ectopic pregnancies and ruptures have been documented at very low β-hCG levels. The emergency medicine guidelines explicitly warn against this pitfall 1.
The ultrasound should assess:
- Intrauterine pregnancy presence and viability (fetal heartbeat, crown-rump length)
- Gestational sac size and characteristics
- Subchorionic hematoma
- Adnexal masses suggesting ectopic pregnancy
- Free fluid in cul-de-sac 3
Laboratory Testing
Obtain β-hCG levels—serial measurements are needed only when ultrasound findings are indeterminate 3. Check:
- Complete blood count for anemia assessment
- Rh factor status (mandatory for potential anti-D immunoglobulin administration)
- Thrombophilia testing only in select cases with relevant history 3
Medical Management
Progesterone Therapy - Evidence-Based Intervention
Vaginal micronized progesterone 400 mg twice daily probably reduces miscarriage risk (RR 0.64,95% CI 0.47-0.87) 4. However, the strength of recommendation depends critically on prior pregnancy history:
- Three or more prior losses: Strongly recommend progesterone (NNT=8) 2
- One or two prior losses: Consider progesterone (NNT=20 for one loss) 2
- No prior losses: Evidence is moderate quality; consider on case-by-case basis 4
Important nuance: Oral progesterone shows benefit (RR 0.57,95% CI 0.38-0.85), but vaginal progesterone alone showed no significant effect (RR 0.75,95% CI 0.47-1.21) in subgroup analysis 4. Despite this, the most recent 2025 guideline specifically recommends vaginal administration 2, likely due to better bioavailability and local uterine effects.
Rh Immunoprophylaxis
Administer 50 µg anti-D immunoglobulin to all Rh-negative women with documented first-trimester pregnancy loss (Level B recommendation) 1. While evidence for threatened abortion without documented loss is insufficient, the theoretical risk of fetomaternal hemorrhage warrants consideration in cases with significant bleeding 1.
Treatments NOT Recommended
Avoid tranexamic acid and bed rest—insufficient data support their use 2. Bed rest, despite being historically prescribed, lacks evidence and may increase thromboembolic risk.
Follow-Up Strategy
Close monitoring is essential given the increased risks documented in threatened abortion 5:
- Preterm delivery risk increases 2-fold (25% vs 12.5%)
- Low birth weight risk more than doubles (23.33% vs 10%)
- Antepartum hemorrhage risk triples (9.17% vs 3.33%)
- NICU admission risk increases 4-fold (28.33% vs 6.67%)
Serial ultrasounds to confirm ongoing viability and growth are warranted, with heightened surveillance for preterm labor signs and fetal growth restriction in the second and third trimesters 5.
Critical Pitfalls to Avoid
- Never defer ultrasound based on "low" β-hCG levels—this is the most dangerous error in threatened abortion management 1
- Don't assume progesterone works for everyone—benefit is most clear with recurrent loss history 2, 4
- Don't forget Rh status—missing anti-D immunoglobulin administration has lifelong consequences for future pregnancies 1
- Don't prescribe unproven therapies—bed rest and tranexamic acid lack supporting evidence 2