Spontaneous Abortion (Threatened)

Threatened abortion is defined as vaginal bleeding through a closed cervical os, with or without abdominal pain, during the first 20 weeks of pregnancy, with a confirmed viable intrauterine pregnancy. [1-2] It occurs in approximately 25% of all pregnancies, and about 10–20% of these will progress to complete miscarriage. [1-2] The pooled risk of miscarriage across all recognized pregnancies is approximately 15.3%. [3]

1. History

• Bleeding characterization: onset, duration, color (bright red vs. brown), volume (spotting vs. pad-soaking), passage of clots or tissue
• Pain: presence, location (midline vs. unilateral), severity, cramping vs. sharp
• Timing: gestational age by LMP, prior ultrasound dating
• Triggers: post-coital bleeding, trauma, exertion
• Associated symptoms: nausea/vomiting (presence is protective — associated with lower miscarriage risk of ~9.7%), dizziness, syncope [4-5]
• Important negatives: passage of tissue (suggests inevitable/incomplete abortion), fever, purulent discharge, unilateral pain (raises concern for ectopic) [6]
• Obstetric history: number of prior pregnancies, prior miscarriages (strongest predictor of recurrence), prior ectopic pregnancies, gestational trophoblastic disease [7-8]

2. Alarm Features

• Hemodynamic instability (tachycardia, hypotension) — suggests hemorrhage or ruptured ectopic [9]
• Heavy bleeding (soaking >1 pad/hour) with clots — increased risk of pregnancy loss (HR 5.03 when combined with cramping) [5]
• Passage of tissue or products of conception visible on exam — indicates inevitable or incomplete abortion [6]
• Open cervical os — abortion is inevitable once the cervix dilates [10]
• Severe unilateral pelvic/abdominal pain — raises concern for ectopic pregnancy [11]
• Peritoneal signs (rebound, guarding) — suggests ruptured ectopic with hemoperitoneum [9]
• Fever with vaginal bleeding — consider septic abortion [12]

3. Medications

• Vaginal micronized progesterone (400 mg BID): recommended for women with early pregnancy bleeding AND a history of ≥1 prior miscarriage; continued to 16 weeks of gestation. The PRISM trial showed a 5% absolute increase in live births in women with bleeding and ≥1 prior miscarriage, and a 15% increase in those with ≥3 prior miscarriages [4][13-14]
• No proven benefit of progesterone in women with threatened miscarriage and no prior miscarriage history [15-16]
• Bed rest: does NOT improve outcomes and is not recommended [17]
• Oral dydrogesterone: not shown to reduce miscarriage rate compared to placebo [18]
• NSAIDs: avoid — associated with increased miscarriage risk [19]
• Rh(D) immune globulin: administer to Rh-negative, unsensitized women, especially later in the first trimester [7]
• Anticoagulants/aspirin: not shown to reduce early pregnancy loss except in antiphospholipid syndrome [7]

The PRISM trial subgroup analysis demonstrated a significant interaction by number of previous miscarriages, as shown in the following figure:

4. Diet

• Caffeine: high intake (>200–300 mg/day) is associated with increased miscarriage risk; advise moderation [21]
• Alcohol: any consumption during pregnancy is associated with increased miscarriage risk; advise complete abstinence [3]
• Hydration: maintain adequate hydration, especially if bleeding is significant
• Folic acid: continue prenatal supplementation (standard of care, though not specifically proven to prevent threatened miscarriage)

5. Review of Systems

• GI: nausea/vomiting (protective sign), diarrhea
• GU: dysuria, frequency (UTI can trigger uterine irritability)
• Constitutional: fever, chills (infection), lightheadedness, syncope (hemorrhage)
• Musculoskeletal: back pain (may accompany cramping)
• Endocrine: symptoms of thyroid disease (thyroid autoantibodies and subclinical hypothyroidism are associated with miscarriage) [3]

6. Collateral History and Family History

• Partner: paternal age >40 years is independently associated with increased miscarriage risk [3]
• Family history: recurrent pregnancy loss, thrombophilias (Factor V Leiden, prothrombin gene mutation), antiphospholipid syndrome, chromosomal rearrangements (balanced translocations in ~2–5% of couples with RPL) [3][22]
• Social context: domestic violence screening, psychosocial stressors (persistent stress is associated with increased risk), occupational exposures (night shifts, pesticides, air pollution) [3]

7. Risk Factors

• Advanced maternal age: risk rises from 12% (age 20–29) → 20% (age 35) → 40% (age 40) → 65% (age ≥45) [3][7]
• Prior miscarriage: risk increases ~10% per additional miscarriage (11% with none → 42% with ≥3) [3]
• Chromosomal abnormalities: account for ~50% of all early pregnancy losses [7]
• Smoking and alcohol [3]
• Obesity (BMI >24.9) or underweight (BMI <18.5) [3]
• Uterine anomalies (septate uterus, fibroids) [3]
• Antiphospholipid syndrome, thyroid autoantibodies, subclinical hypothyroidism [3]
• Infections: bacterial vaginosis, Chlamydia, TORCH infections [3]
• Poorly controlled diabetes [21]

8. Differential Diagnosis

• Ectopic pregnancy — cannot-miss diagnosis; 1–2% of pregnancies, accounts for 6% of maternal deaths. Unilateral pain, β-hCG above discriminatory zone without visible IUP [11][17]
• Inevitable/incomplete abortion — open cervical os, passage of tissue
• Missed abortion — no fetal cardiac activity, closed os, no active bleeding
• Molar pregnancy (gestational trophoblastic disease) — "snowstorm" ultrasound pattern, markedly elevated β-hCG [7]
• Cervical pathology — cervicitis, cervical polyp, cervical ectropion (post-coital bleeding)
• Subchorionic hemorrhage — common ultrasound finding; associated with increased but not definitive risk of loss [7]
• Implantation bleeding — typically light, self-limited, occurs around expected menses
• Ovarian torsion, ruptured corpus luteum cyst — unilateral pain, may have free fluid [11]

9. Past Medical History

• Prior miscarriages (number, gestational age, any workup performed)
• Prior ectopic pregnancies
• Uterine surgery (myomectomy, D&C, cesarean section — cervical trauma from LEEP increases mid-trimester loss risk) [23]
• Autoimmune conditions (antiphospholipid syndrome, SLE)
• Endocrine disorders (thyroid disease, PCOS, diabetes)
• Thrombophilias
• History of assisted reproductive technology

10. Physical Exam

• Vitals: heart rate, blood pressure (assess for hemodynamic instability), temperature (fever → infection)
• Abdominal exam: tenderness (diffuse vs. localized/unilateral), peritoneal signs, uterine size relative to dates (discrepancy ≥4 weeks is a predictor of nonviable pregnancy) [6]
• Speculum exam (critical):
  • Assess cervical os — closed os = threatened abortion; open os = inevitable abortion [1][10]
  • Quantify bleeding, identify source (cervical vs. uterine)
  • Look for visible tissue/products of conception in the os or vaginal vault
  • Evaluate for cervical lesions, polyps, lacerations
• Bimanual exam: uterine size, tenderness, adnexal masses or tenderness (ectopic)

11. Lab Studies

• Quantitative β-hCG: essential for establishing pregnancy viability and trending
  • Normal rise: ≥49% increase over 48 hours (for initial hCG <1,500 mIU/mL) [24]
  • Discriminatory level: 1,500–3,000 mIU/mL — IUP should be visible on TVUS above this level [17][25]
  • Abnormal rise or plateau raises concern for ectopic or nonviable pregnancy
• Blood type and Rh status: for Rh(D) immune globulin decision
• CBC: assess for anemia if significant bleeding
• Serum progesterone: low levels (<35 nmol/L) are the strongest predictor of miscarriage in threatened abortion (OR 26.3) [26-27]
• TSH: if thyroid disease suspected
• Consider coagulation studies if heavy bleeding or history of bleeding disorder

12. Imaging

• First-line: Transvaginal ultrasound (TVUS) combined with transabdominal US — the preferred modality for evaluating first-trimester bleeding [7][28]
  • Confirm intrauterine pregnancy location
  • Assess fetal cardiac activity
  • Evaluate for subchorionic hemorrhage
• Diagnostic criteria for pregnancy loss (ACOG/Society of Radiologists in Ultrasound): [4][28]
  • CRL ≥7 mm with no cardiac activity = diagnostic of pregnancy loss
  • Mean sac diameter ≥25 mm with no embryo = diagnostic of pregnancy loss
  • CRL <7 mm without cardiac activity = suspicious → repeat US in 7–10 days
• Suggestive but not diagnostic findings: slow fetal heart rate (<100 bpm at 5–7 weeks), enlarged yolk sac (>7 mm), subchorionic hemorrhage [7][28]
• MRI pelvis without contrast: may be appropriate when US is limited by habitus or nontubal ectopic is suspected [28]
• CT: generally avoided due to ionizing radiation; reserved for suspected surgical emergency

13. Special Tests

• Serial β-hCG (48-hour interval): for pregnancy of unknown location or uncertain viability [24]
• Serum progesterone: single level can serve as triage tool; ≥35 nmol/L associated with 9.6% miscarriage rate vs. 70.8% when <35 nmol/L [27]
• Point-of-care ultrasound (POCUS): in the ED, can rapidly identify IUP, free fluid, or adnexal mass
• Rh antibody screen: in Rh-negative patients
• Recurrent pregnancy loss workup (if ≥2 losses): antiphospholipid antibodies (lupus anticoagulant, anticardiolipin, anti-β2 glycoprotein I), parental karyotyping, uterine cavity evaluation [22]

14. ECG

• Not routinely indicated
• Consider if hemodynamically unstable, significant hemorrhage, or pre-procedural assessment
• Evaluate for tachycardia patterns if concern for hypovolemic shock

15. Assessment

Threatened abortion is a clinical diagnosis defined by vaginal bleeding with a closed cervix and a confirmed viable intrauterine pregnancy. [1-2] The key clinical priorities are:

• Confirm intrauterine pregnancy and viability (rule out ectopic — the most dangerous diagnosis to miss)
• Assess hemodynamic stability
• Risk-stratify based on bleeding severity, prior miscarriage history, maternal age, and ultrasound findings

Approximately 50% of women with threatened abortion will carry to term. [2] Heavy bleeding combined with cramping carries the highest risk of loss (cumulative incidence ~81%). [5] Threatened miscarriage is also associated with later pregnancy complications including preterm delivery (OR 2.05), fetal growth restriction (OR 1.54), and placental abruption. [3]

16. Treatment Plan

Initial stabilization:

• IV access and fluid resuscitation if hemodynamically unstable
• Type and screen; crossmatch if significant hemorrhage

Expectant management is the standard treatment for threatened abortion with confirmed viable IUP: [17]

• No specific intervention has been proven to prevent progression in women without prior miscarriage history
• Bed rest is NOT recommended — no evidence of benefit [17]

Progesterone supplementation (for eligible patients): [4][13]

• Vaginal micronized progesterone 400 mg BID, started at onset of bleeding, continued to 16 weeks
• Indicated in women with early pregnancy bleeding AND ≥1 prior miscarriage
• NNT = 18 for one additional live birth (in women with ≥1 prior miscarriage) [4]

Supportive care:

• Analgesics (acetaminophen preferred; avoid NSAIDs)
• Rh(D) immune globulin if Rh-negative [7]
• Emotional support and counseling — miscarriage causes significant anxiety, depression, and PTSD symptoms [1]

17. Disposition

Discharge criteria (majority of threatened abortions are managed outpatient):

• Hemodynamically stable
• Confirmed viable IUP on ultrasound
• Bleeding is not heavy/life-threatening
• Ectopic pregnancy excluded
• Reliable follow-up arranged

Admission criteria:

• Hemodynamic instability or significant hemorrhage
• Suspected ectopic pregnancy requiring urgent intervention
• Severe pain uncontrolled with oral analgesics
• Inability to arrange timely follow-up
• Signs of infection (septic abortion)

Observation indications:

• Pregnancy of unknown location with β-hCG near discriminatory zone
• Uncertain viability requiring serial assessment

Specialist consultation triggers:

• OB/GYN consultation for any hemodynamically unstable patient, suspected ectopic, or need for surgical intervention
• Maternal-fetal medicine referral for recurrent pregnancy loss workup

18. Follow Up / Return Precautions

Follow-up timing:

• OB/GYN or early pregnancy unit follow-up within 48–72 hours for repeat β-hCG and/or ultrasound if viability uncertain
• If viable IUP confirmed, follow-up in 7–14 days for repeat ultrasound to reassess viability [7]
• Routine prenatal care if pregnancy continues

Return precautions — instruct patient to return immediately for:

• Heavy bleeding (soaking >1 pad/hour for ≥2 hours)
• Passage of tissue or clots
• Severe or worsening abdominal/pelvic pain
• Dizziness, lightheadedness, or syncope
• Fever or chills
• Shoulder pain (referred pain from diaphragmatic irritation — ruptured ectopic)

Patient counseling:

• Light spotting may continue and does not necessarily indicate worsening
• Avoid intercourse, tampons, and douching until bleeding resolves and cleared by provider
• Continue prenatal vitamins
• Once fetal heart activity is confirmed at 8 weeks, the likelihood of live birth approaches 97–98% [3]
• Emotional support resources; miscarriage is associated with significant psychological morbidity [3]

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