What are the transvaginal ultrasound criteria for diagnosing an interstitial (cornual) pregnancy and the recommended management for a hemodynamically stable patient?

Interstitial Pregnancy: Ultrasound Diagnosis and Management

An interstitial (cornual) pregnancy should be suspected on transvaginal ultrasound when a gestational sac is located within 5-7 mm of the myometrial edge, appears eccentric in position, or shows a protruding cornual mass—and for hemodynamically stable patients, immediate gynecology consultation is mandatory given the 2-4% maternal mortality risk associated with this condition. 1

Transvaginal Ultrasound Diagnostic Criteria

Primary Sonographic Features

The key diagnostic finding is a gestational sac positioned within 5-7 mm from the edge of the myometrium, which distinguishes interstitial pregnancy from normal intrauterine implantation 1. This measurement represents the critical threshold that should raise immediate concern.

Additional diagnostic features include:

• Eccentric sac location at the uterine fundus or cornual region, rather than the central endometrial cavity 2, 3
• "Pregnancy in cornus" appearance—a gestational sac or mass protruding from the cornual area 2
• Empty endometrial cavity with an endometrial echo present, but no central gestational sac 2, 3
• Thin myometrial mantle surrounding the gestational sac (often described as "surrounded by thin myometrium") 4, 3
• Increased vascularity at the cornual region on Doppler examination 4

Technical Scanning Approach

The uterus must be examined in at least two orthogonal planes (short and long axis) to avoid missing interstitial pregnancies that lie off the central endometrial canal 1. The entire uterus should be traced systematically from fundus to cervix to confirm you are actually scanning the uterus rather than a large ectopic gestational reaction 1.

Critical Pitfall Recognition

Interstitial pregnancy is explicitly listed as a diagnostic pitfall in emergency ultrasound guidelines, meaning it is frequently missed even by experienced operators 1. The condition can be confused with:

• Normal eccentric implantation in early pregnancy
• Cornual fibroids
• Angular pregnancy (which implants laterally but within the endometrial cavity)

Management for Hemodynamically Stable Patients

Immediate Actions

Obtain immediate gynecology or reproductive endocrinology consultation as soon as interstitial pregnancy is suspected on ultrasound, regardless of β-hCG level or gestational age 4, 5. This is not a condition for emergency medicine or primary care management alone.

Document hemodynamic stability carefully: blood pressure, heart rate, orthostatic vital signs, and severity of abdominal pain 6, 7. Interstitial pregnancies can rupture catastrophically even when patients initially appear stable 8, 5.

Treatment Options (Specialist-Directed)

For hemodynamically stable patients with early diagnosis, three management pathways exist:

Medical management with methotrexate may be attempted when:

• Gestational age is very early (typically <8 weeks)
• β-hCG levels are not excessively elevated
• No fetal cardiac activity is present
• Patient can comply with close follow-up

However, medical management has a significant failure rate in interstitial pregnancy, as demonstrated by the case requiring surgical intervention after failed methotrexate 4.

Conservative surgical approaches (cornuostomy without cornual resection) are increasingly preferred for fertility preservation:

• Hysteroscopy-assisted laparoscopic aspiration of the gestational sac 4
• Laparoscopic cornuostomy with intramyometrial vasopressin to reduce bleeding 5
• These techniques preserve uterine architecture and future reproductive potential 4, 5

Cornual resection (cornuectomy) remains necessary when:

• Rupture has occurred or is imminent
• Gestational age is advanced
• β-hCG levels are very high (>10,000-15,000 mIU/mL)
• Conservative approaches have failed 5, 3

Monitoring Protocol

Serial β-hCG measurements every 48 hours are essential even in stable patients to assess for appropriate decline with medical management or to detect treatment failure 6, 7. After successful medical management, β-hCG should decline significantly within 5-7 days 4.

Repeat transvaginal ultrasound should be performed if β-hCG plateaus or rises, or if clinical symptoms worsen 9, 6.

Critical Safety Considerations

Warning Signs Requiring Immediate Surgical Intervention

Patients must return immediately for emergency evaluation if they develop:

• Severe or worsening unilateral abdominal pain (suggests impending or actual rupture) 6, 7
• Shoulder pain (indicates hemoperitoneum from rupture) 6
• Dizziness, syncope, or hemodynamic instability (suggests significant hemorrhage) 6, 3
• Heavy vaginal bleeding 6

Why Interstitial Pregnancy Is Uniquely Dangerous

Interstitial pregnancy carries a 2-4% maternal mortality rate—significantly higher than other ectopic locations—because:

• The cornual region is highly vascular with rich blood supply 8, 5
• Rupture typically occurs later (8-16 weeks) when the gestational sac is larger 8, 5
• Rupture results in catastrophic hemorrhage into the peritoneal cavity 8, 3
• The myometrium at the cornual region is thinner and more prone to rupture 2, 3

Future Pregnancy Considerations

Patients with prior interstitial pregnancy require:

• Counseling about increased risk of repeat interstitial pregnancy in future conceptions 5
• Elective cesarean delivery in subsequent pregnancies to avoid uterine rupture at the cornuostomy site 5
• Early ultrasound in future pregnancies to confirm normal implantation location 5

Common Diagnostic Errors to Avoid

Never defer ultrasound based on β-hCG level alone—interstitial pregnancies can present at any β-hCG value, and approximately 22% of all ectopic pregnancies occur with β-hCG <1,000 mIU/mL 6, 7.

Do not assume a visible "intrauterine" gestational sac rules out ectopic pregnancy if the sac appears eccentric or is located in the cornual region—this may actually represent the interstitial pregnancy itself 2, 3.

Avoid relying solely on the discriminatory threshold (3,000 mIU/mL) to guide management decisions, as this threshold has poor diagnostic utility for ectopic pregnancy detection (positive likelihood ratio 0.8) 9, 6.

Do not mistake free fluid in the cul-de-sac as a normal finding—in the context of suspected ectopic pregnancy, significant free fluid raises concern for rupture 1, 2.