The Development of an Ultrasound-Based Scoring System for the Prediction of Interstitial Pregnancy

Abstract

Background/Objectives: Diagnosing interstitial pregnancy (IP) using ultrasonography can be challenging, as it is often mistaken for eccentrically located intrauterine pregnancy (IUP). In this retrospective cohort study, we aimed to develop a predictive scoring model using multiple clinical factors to enhance the diagnosis of IP and facilitate timely interventions in suspected cases. Methods: We enrolled 63 pregnant women with a diagnosis of suspected IP who visited a single tertiary center between January 2006 and December 2023. Data on the clinical risk factors, symptoms, laboratory test results, and ultrasound findings were analyzed. A statistical predictive score was developed using logistic regression analysis with feature selection based on the least absolute shrinkage and selection operator to optimize the predictive accuracy and clinical applicability. Results: From a total of 12 factors, a scoring model was constructed from the three most prominent factors-ultrasound findings showing no surrounding endometrium, myometrial thinning of less than 5 mm, and vaginal bleeding-all of which demonstrated high feature importance. This predictive score identified IP with a negative predictive value of 0.950 in the low-risk group and a positive predictive value of 1.000 in the high-risk group, whereas the overall area under the curve was 0.998 (95% confidence interval, 0.992-1.000). Conclusions: The statistically derived predictive model--ultrasound showing no surrounding endometrium and myometrial thinning < 5 mm combined with vaginal bleeding--demonstrated high accuracy and practical applicability for IP diagnosis, providing a robust tool to enhance clinical decision-making and optimize routine management strategies for IP.

Keywords: clinical decision-making; cornual pregnancy; ectopic pregnancy; interstitial pregnancy; predictive value of tests; ultrasonography.

Figure 1

Representative US images of IP. (a) Eccentrically located round ring-like mass (white arrowhead) in the right uterine cornu in the transverse plane. (b) Asymmetric, thin myometrial (<5 mm) layer surrounding the GS (black arrowhead), without surrounding endometrium. (c) Empty uterine cavity and interstitial portion of the tube extending from the uterine cavity to the GS (interstitial line sign, white arrow). GS, gestational sac; IP, interstitial pregnancy; US, ultrasound.

Figure 2

Study population.

Figure 3

Decision tree model for predicting IP diagnosis.