Ruptured Ectopic Pregnancy
The most likely diagnosis is D. Ruptured ectopic pregnancy. This patient's presentation of sudden severe abdominal pain radiating to the right shoulder, six-week amenorrhea (irregular cycle with last period six weeks ago), and abundant free fluid in the pouch of Douglas on ultrasound creates a classic triad for ruptured ectopic pregnancy 1.
Key Diagnostic Features Supporting Ruptured Ectopic Pregnancy
The abundant fluid in the pouch of Douglas is the critical finding that distinguishes this from other pregnancy complications. 1 This fluid represents hemoperitoneum from tubal rupture, and when combined with a positive pregnancy test (implied by the clinical scenario), it is highly concerning for ruptured ectopic pregnancy 1.
- The right shoulder pain (Kehr's sign) indicates diaphragmatic irritation from blood in the peritoneal cavity, a pathognomonic feature of intraperitoneal hemorrhage 1
- Six-week amenorrhea places the patient at the typical gestational age (6-8 weeks) when ectopic pregnancies rupture, as the growing pregnancy stretches the fallopian tube 1, 2
- The sudden onset of severe pain is characteristic of acute rupture, rather than the gradual pain of other conditions 1
Why Other Diagnoses Are Unlikely
Septic Miscarriage (Option A)
- Septic miscarriage requires fever, purulent vaginal discharge, and systemic signs of infection (tachycardia, hypotension from sepsis) 1
- The patient's presentation lacks these infectious symptoms
- Septic miscarriage would not cause abundant free fluid in the pouch of Douglas 1
Appendicular Mass (Option B)
- Appendicitis does not explain the six-week amenorrhea or positive pregnancy status
- An appendicular mass would not produce abundant free fluid in the pouch of Douglas 3
- Right shoulder pain is not a feature of appendicitis
Incomplete Miscarriage (Option C)
- Incomplete miscarriage presents with cramping pain and heavy vaginal bleeding with passage of tissue 1
- It would not cause abundant free fluid in the pouch of Douglas 1
- The sudden severe pain and shoulder radiation are inconsistent with incomplete miscarriage
Diagnostic Confirmation Steps
Immediate management should focus on hemodynamic stabilization while confirming the diagnosis:
- Obtain serum β-hCG level: An elevated β-hCG above 3,000 mIU/mL with no intrauterine pregnancy on ultrasound strongly suggests ectopic pregnancy 1
- Complete blood count: Assess for anemia from hemorrhage 1
- Type and crossmatch: Prepare for potential blood transfusion 1
- Transvaginal ultrasound findings to look for: Absence of intrauterine gestational sac, adnexal mass or tubal ring, and free fluid with internal echoes (blood) 2
Critical Clinical Pitfall
Do not delay surgical consultation based on β-hCG levels alone. 2 Approximately 22% of ectopic pregnancies occur at β-hCG levels below 1,000 mIU/mL, and the presence of abundant free fluid with hemodynamic compromise requires immediate surgical intervention regardless of β-hCG value 4, 2. The combination of clinical presentation (shoulder pain, sudden severe abdominal pain) with ultrasound findings (abundant free fluid) in a patient with amenorrhea is sufficient to proceed with emergency laparoscopy or laparotomy 1, 5.