Accuracy of first trimester sonographic features in diagnosing caesarean scar pregnancy

Caesarean scar pregnancy (CSP), if not managed timely, can result in pre‐term labour, postpartum haemorrhage, hysterectomy or maternal and fetal death. This study aims to compare the diagnostic performances of different sonographic features recommended in various good practice recommendations and literature; then propose an algorithm that could aid in easier identification of CSP using transvaginal ultrasound.A retrospective review of ultrasound images of intrauterine pregnancy (IUP) and CSP between 4.9 and 11.9 weeks gestation was conducted. The diagnostic performance of the Royal College of Obstetricians & Gynaecologists (RCOG) guideline, Timor‐Tritsch et al. (TT) method and combined method were evaluated. An algorithm was then created to calculate a numerical value for diagnosis. The methods were tested for intra‐ and inter‐observer agreement.A total of 66 ultrasound cases (30 IUP and 36 CSP) were included in this study. The sensitivity of the RCOG guideline, TT method and combined method were 58.3%, 94.4% and 100% while the specificity were 96.7%, 90% and 90%, respectively. The TT method and combined method performed significantly better than the RCOG guideline (p = .0016 and .0002, respectively). Using the proposed algorithm, a value larger or equal to 52.2 gave 97.2% sensitivity and 93.3% specificity that CSP is present. Intra‐ and inter‐observer agreement for all methods were moderate to almost perfect.Screening during early first trimester is important to improve the diagnostic accuracy of CSP. The TT method is a good supplement to the RCOG guideline. The proposed algorithm is a useful tool to improve diagnostic accuracy and confidence.