CASE 18328 Published on 10.10.2023

Interstitial pregnancy: MRI and US


Genital (female) imaging

Case Type

Clinical Cases


Brittany Le, Angelina Lo, Sonia Lee

University of California Irvine Medical Center, Orange, California, United States of America


33 years, female

Area of Interest Abdomen, Anatomy, Emergency, Genital / Reproductive system female, Obstetrics (Pregnancy / birth / postnatal period) ; No Imaging Technique
Clinical History

A 33-year-old female with a history of miscarriage within the last 12 months presented to the Emergency Department at 7 weeks pregnant for vaginal spotting that started two weeks prior.

She reported intermittent nausea and abdominal cramping, but was afebrile with vital signs stable.

Imaging Findings

Transvaginal ultrasound was performed. It demonstrated an anechoic structure with a hyperechoic rim high in the left cornua with thinning of the overlying myometrium to 2 mm (Figure 2, Video 1, Video 2 and Video 3) consistent with a gestational sac, with partially visualized fetus and yolk sac (Figure 1a). The fetal crown rump length was measured at 1.79 cm (Figure 1b).

Magnetic resonance imaging (MRI) was also performed in order to confirm the location of the sac, showing an eccentrically located fluid-filled structure high in the left cornua with a bulging contour (Figures 3a and 3b).



Ectopic pregnancy is the implantation of a blastocyst outside the endometrial uterine cavity. Less commonly, it implants within the proximal intramural portion, termed interstitial pregnancy. This term was historically used interchangeably with cornual pregnancy, but cornual pregnancy refers to implantation within a septate or bicornuate uterus. Implantation can also occur at the lateral angle of the uterine cavity called angular pregnancy, distinguished from interstitial pregnancy by medial placement to the round ligament. Interstitial pregnancies account for about 2-4% of ectopic pregnancies [1]. Major risk factors of ectopic pregnancy include intrauterine instrumentation, pelvic inflammatory disease, and assisted reproductive techniques [2].

The classic symptoms of ectopic pregnancy are abdominal pain and vaginal bleeding, commonly diagnosed around 6-8 weeks gestation [1,3]. If not removed, interstitial pregnancies usually rupture in the second trimester. First-line evaluation is with non-invasive transvaginal ultrasound as it establishes the gestation location, age, and associated complications for the requesting physician.

Imaging Perspective

Transvaginal ultrasound diagnosis relies upon three criteria: eccentric position of the gestational sac less than 1 cm from the most lateral edge of the uterine cavity, a thin (less than 5mm) myometrial layer surrounding the sac, and an empty uterus [1,4]. The thin myometrial layer is termed the endomyometrial mantle [4]. Early in pregnancy, the gestational sac is located laterally in the uterus but can be seen above the fundus as the pregnancy advances. This is termed the “interstitial line sign”, an echogenic line that extends centrally upwards indicating the interstitial fallopian tube [5].

When it is difficult to visualize the exact location of an ectopic pregnancy, MRI may be helpful in confirming the implantation site. MRI has superior tissue contrast imaging, improving visualization of anatomy. On MRI, the gestational sac should be seen eccentric to the junctional zone. MRI allows for better evaluation of the outer contour of the uterus and outer bulging which is used intra-operatively to diagnose interstitial implantation [6].


Initial treatment focuses on preservation of the uterus through injection of potassium chloride and methotrexate or cornuostomy [1,2]. Hysterectomy with resection is indicated in persistent bleeding [2]. It is important to diagnose early, as gestational age and hemodynamic stability determine whether conservative treatment can be attempted. Close follow-up is necessary, and subsequent pregnancies should be monitored with transvaginal ultrasound 5-6 weeks after the last menstrual period [1]. In this case, identification of interstitial pregnancy allowed for cornuostomy to be performed, preserving fertility.

All patient data have been completely anonymised throughout the entire manuscript and related files.

Differential Diagnosis List
Ovarian torsion
Normal intrauterine gestation
Pelvic inflammatory disease
Threatened abortion
Interstitial ectopic pregnancy
Angular pregnancy
Final Diagnosis
Interstitial ectopic pregnancy
Case information
DOI: 10.35100/eurorad/case.18328
ISSN: 1563-4086