What is the interstitial line sign and how does it help diagnose an interstitial (cornual) pregnancy?

Interstitial Line Sign

Definition and Sonographic Appearance

The interstitial line sign is a thin echogenic line visible on transvaginal ultrasound that connects the endometrium to an ectopic gestational sac, representing the interstitial segment of the fallopian tube, and serves as a specific diagnostic marker for interstitial ectopic pregnancy. 1

• The sign appears as a thin echogenic line extending from the endometrial cavity to the gestational sac located in the interstitial (intramyometrial) portion of the fallopian tube 1
• This echogenic line represents the actual interstitial segment of the tube itself, providing anatomic confirmation of the pregnancy's location 1

Diagnostic Utility

While the interstitial line sign is useful for confirming interstitial ectopic pregnancy, it is rarely encountered in clinical practice and should not be relied upon as the primary diagnostic criterion. 1

• The Society of Radiologists in Ultrasound consensus guidelines explicitly state that "the interstitial line sign may be useful but is rarely encountered" 1
• When present, it provides additional confirmatory evidence beyond the primary diagnostic features of interstitial ectopic pregnancy 1

Primary Diagnostic Criteria for Interstitial Ectopic Pregnancy

Since the interstitial line sign is infrequently visualized, diagnosis should rely on these more consistently identifiable features:

• Empty uterine cavity with gestational sac located >1 cm from the endometrial cavity 2
• Intervening myometrium between the gestational sac and endometrium 1
• Thin myometrial mantle surrounding the gestational sac (<5 mm) 2
• Claw sign of myometrial tissue partially surrounding the gestational sac 1
• Eccentric location of the gestational sac at the cornual area 2, 3

Clinical Significance

Accurate identification of interstitial ectopic pregnancy is critical because the mortality rate is twice that of other ectopic pregnancies, making early diagnosis essential to prevent life-threatening rupture and hemorrhage. 2, 4

• Interstitial pregnancies account for only 1-3% of all ectopic pregnancies but carry disproportionately high maternal morbidity and mortality risk 4
• The thicker myometrial mantle allows these pregnancies to progress later before rupture, increasing the risk of catastrophic hemorrhage 4
• Misdiagnosis as an eccentric intrauterine pregnancy can lead to inappropriate expectant management, as eccentric intrauterine pregnancies may have live birth rates up to 69% 2, 5

Complementary Diagnostic Tools

• Three-dimensional coronal reformatted ultrasound may better demonstrate the separation between the interstitial gestational sac and endometrium 1
• Absence of sliding sign between the uterus and gestational sac helps confirm implantation rather than a passing pregnancy 1, 6
• Transvaginal ultrasound is superior to transabdominal imaging for identifying the exact location of early gestational sacs 3, 4

Important Caveats

• Do not wait for the interstitial line sign to appear before making the diagnosis, as it is an inconsistent finding 1
• Early first-trimester ultrasound (preferably by 7-8 weeks) is essential to differentiate true interstitial ectopic pregnancy from eccentric intrauterine pregnancy before clinical deterioration occurs 4, 5
• The term "cornual pregnancy" should be avoided in favor of "interstitial ectopic pregnancy" for standardized terminology 1