Easy sonographic differential diagnosis between intrauterine pregnancy and cesarean delivery scar pregnancy in the early first trimester

BACKGROUND: Cesarean scar pregnancy (CSP) is a serious complication of pregnancy, which consists of implantation of the gestational sac in the hysterotomy scar. This condition is increasing in frequency and often poses a diagnostic challenge. Its diagnosis is dependent on visual assessment of the uterus on the longitudinal sagittal ultrasound plane. Misdiagnosing a low intrauterine chorionic sac as a CSP, or a true scar pregnancy as an intrauterine pregnancy (IUP), may lead to adverse outcomes including hysterectomy. OBJECTIVE: The objective of the study is to describe a sonographic method for the differential diagnosis of CSP vs IUP in early gestation. The current study tests the hypothesis that on a first-trimester ultrasound performed between 5-10 weeks of gestation, the relative location of the center of gestational sac to the midpoint of the uterus along a longitudinal line between the external cervical os and the fundus can be used for early detection of CSPs. STUDY DESIGN: This is a retrospective review of electronically archived ultrasound images of IUP and CSP between 5-10 weeks of gestation. A total of 242 ultrasound images were analyzed: 185 cases of normal IUPs (including 128 in anteverted uteri, 31 in retroverted uteri, and 26 IUPs with history of cesarean delivery) and 57 cases of CSPs diagnosed from 2004 through 2015 in a single institution. The following measurements were made for each case: distance from the external cervical os to the uterine fundus, the midpoint axis of the uterus, the distance from the external cervical os to the center of gestational sacs, and the distance from the external cervical os to the most distant edge of the gestational sacs from the cervix. RESULTS: The location of the center of the gestational sac relative to the midpoint axis of the uterus between 5-10 weeks of gestation differentiated between IUP and CSP (mean 17.8 vs -10.6 mm, respectively, P = .0001), indicating that most CSPs are located proximally to the midpoint axis of the uterus whereas most normal IUPs are located distally from the midpoint of the uterus. Using location of the center of the gestational sac as a marker of CSPs between 5-10 weeks of gestation yielded the following characteristics of diagnostic accuracy: sensitivity 93.0% and specificity 98.9%. The likelihood ratio of the positive test was 84.5. The likelihood ratio of the negative test was 0.07. CONCLUSION: The location of the center of the gestational sac relative to the midpoint axis of the uterus can be used as an easy method for sonographic differentiation of IUP and CSP between 5-10 weeks of gestation.