Interstitial ectopic pregnancy is implantation of the blastocyst in the intramural (interstitial) segment of the fallopian tube — the ~1-2 cm portion passing through the myometrium. It comprises 2-4% of all ectopic pregnancies but the mortality rate is 2-5 times higher than tubal ectopics because the surrounding myometrium allows the pregnancy to grow to later gestational age, and when rupture occurs, massive hemorrhage develops due to the dense anastomotic network of uterine artery branches in the cornual region. On US, eccentric cornual sac, thin myometrial mantle (<5 mm), and interstitial line sign are characteristic. TVUS is the primary diagnostic modality with over 80% sensitivity; MRI is utilized in equivocal cases and for accurate measurement of myometrial mantle thickness.
Age Range
18-45
Peak Age
32
Gender
Female predominant
Prevalence
The interstitial segment is the ~1-2 cm portion of the tube passing through the uterine wall, surrounded by myometrium. When the blastocyst implants in this segment, the myometrial wall thickness (~5 mm) forms a thin mantle. Due to myometrial muscular strength, the pregnancy can grow up to 12-16 weeks — much later rupture than tubal ectopics. The anatomic position of the interstitial segment provides proximity to arcuate artery anastomoses of the uterine artery branches; therefore intense neovascularization develops during trophoblastic invasion, and when rupture occurs, this extensive vascular network makes bleeding uncontrollable. On US, the interstitial line sign is observed: a thin echogenic line between the gestational sac and endometrial cavity — this line represents the proximal portion of the interstitial tubal segment and proves the extra-cavitary position. Myometrial mantle <5 mm correlates with increased rupture risk; this threshold is based on clinical studies and is determinative in treatment decisions. As pregnancy progresses, trophoblastic invasion thins the myometrial wall and eventually reaches the serosal surface leading to rupture.
Thin echogenic line between cornual gestational sac and endometrial cavity — represents the proximal end of the interstitial tubal segment and is considered pathognomonic for interstitial ectopic pregnancy. Has over 80% sensitivity and plays a critical role in differential diagnosis.
An eccentrically located gestational sac is seen in the uterine cornu on TVUS — the sac is separated laterally from the endometrial cavity, surrounded by myometrium. Bulging of the sac beyond the uterine contour may indicate advanced interstitial ectopic pregnancy. Sac diameter is typically 15-40 mm and correlates with gestational age.
Report Sentence
An eccentrically located gestational sac in the uterine cornu is consistent with interstitial ectopic pregnancy.
A thin echogenic line (interstitial line sign) is seen between the gestational sac and endometrial cavity. This line represents the proximal (uterine) end of the interstitial tubal segment and proves the interstitial position of the sac. Has over 80% sensitivity. The line length is typically 5-10 mm and reflects the distance of the sac from the cavity.
Report Sentence
Interstitial line sign is seen between the sac and endometrial cavity, supporting the diagnosis of interstitial ectopic pregnancy.
Myometrial thickness (mantle) surrounding the gestational sac measures <5 mm. Normal myometrial thickness is >5 mm; thinning indicates increased rupture risk. Measurement is taken at the thinnest myometrial covering — usually lateral/superior wall. When mantle thickness drops below 3 mm, imminent rupture risk increases significantly.
Report Sentence
Myometrial mantle surrounding the sac measures _ mm, requiring assessment for rupture risk.
On T2-weighted MRI, a gestational sac (high T2 signal — fluid) surrounded by myometrium is seen in the uterine cornu. Myometrium shows intermediate signal, sac fluid high signal → clear contrast. Interstitial segment continuity and myometrial mantle thickness can be more accurately evaluated on MRI than US, especially in advanced gestational age. Sagittal and coronal planes allow detailed evaluation of the relationship between uterine cavity and cornual sac.
Report Sentence
A gestational sac surrounded by myometrium in the uterine cornu is seen on MRI, consistent with interstitial ectopic pregnancy.
Intense peritrophoblastic vascularity is seen around the cornual sac on color Doppler — similar to ring-of-fire pattern. Myometrial branches and uterine artery arcuate anastomoses are prominent. Intense vascularity reflects massive bleeding potential in case of rupture. Spectral Doppler demonstrates low-resistance arterial flow pattern (RI <0.4).
Report Sentence
Intense peritrophoblastic vascularity is seen around the cornual sac on color Doppler.
Contralateral uterine cornu shows normal anatomic structure without gestational sac on TVUS. This asymmetry highlights the abnormal position of the sac in the affected cornu and confirms absence of bilateral cornual pregnancy. Normal appearance of the contralateral cornu also supports unilateral interstitial ectopic pregnancy diagnosis and helps differentiate from müllerian anomalies such as bicornuate uterus.
Report Sentence
Contralateral uterine cornu shows normal structure without gestational sac.
Hyperintense signal areas on T1-weighted MRI are seen within or around the myometrial mantle — reflecting subacute hemorrhage or hemosiderin deposition. These findings may indicate impending rupture or local hemorrhage from micro-rupture. Post-contrast T1 sequences demonstrate prominent enhancement of trophoblastic tissue confirming interstitial pregnancy.
Report Sentence
Hyperintense signal areas around the myometrial mantle on T1-weighted MRI are consistent with subacute hemorrhage.
Criteria
Myometrial mantle intact, no hemoperitoneum, patient hemodynamically stable
Distinct Features
Medical (MTX) or conservative surgical treatment possible — MTX preferred if β-hCG <5000 and mantle >5 mm; close US and β-hCG monitoring mandatory
Criteria
Myometrial mantle ruptured, hemoperitoneum present, hemodynamic instability
Distinct Features
EMERGENT surgery mandatory, massive hemorrhage risk, cornuectomy/cornual resection may be needed, high mortality, uterine artery embolization may help control bleeding
Criteria
Myometrial mantle very thin (<3 mm) but not yet ruptured, focal bulging present
Distinct Features
Emergent surgical intervention should be planned, MTX contraindicated (very high rupture risk), laparoscopic cornuostomy or cornual resection preferred
Distinguishing Feature
In tubal ectopic, sac is in the adnexal region separate from uterus without myometrial covering; in interstitial, sac is within the myometrium in the uterine cornu with interstitial line sign
Distinguishing Feature
Cesarean scar ectopic sac is at the anterior lower segment scar associated with myometrial defect; interstitial sac is in the cornual region with positive interstitial line sign
Distinguishing Feature
Cervical ectopic sac is within cervix below internal os with barrel-shaped cervical expansion; interstitial sac is within cornual myometrium with cornual bulging
Distinguishing Feature
In angular pregnancy, sac is at the uterine cavity corner but shows continuity with endometrial cavity without interstitial line sign; in interstitial ectopic, sac is separate from cavity with positive interstitial line sign
Urgency
emergentManagement
surgicalBiopsy
Not NeededFollow-up
specialist-referralInterstitial ectopic carries high mortality risk — rupture can result in massive hemoperitoneum. In unruptured cases, methotrexate (if β-hCG <5000 and mantle >5 mm) or laparoscopic cornuostomy/cornual resection is performed. Ruptured cases may require emergent laparotomy + cornuectomy/hysterectomy. Uterine artery embolization may help control bleeding. Post-treatment β-hCG monitoring continues until negative. Future pregnancies carry risk of uterine rupture and require close monitoring.
Interstitial ectopic pregnancy presents later than tubal ectopic (myometrial mantle allows expansion) but mortality is much higher when rupture occurs (uterine artery branch hemorrhage). Early diagnosis and treatment are life-saving. Treatment options include methotrexate (small, unruptured), laparoscopic cornuotomy/cornual resection, or hysterectomy (in case of rupture).