Cesarean scar ectopic pregnancy occurs when an embryo implants into or on a prior cesarean scar, and carries a high burden of maternal morbidity and increased risk of maternal mortality when left untreated.¹ The estimated incidence is approximately 1 in 1800–2000 pregnancies after cesarean delivery.² The key to prompt treatment is early identification, most commonly and easily accomplished with transvaginal ultrasound.

Cesarean delivery is the most commonly performed surgery in the world. In the United States, according to the Centers for Disease Control’s Stats of the States data from 2021,  the cesarean delivery rate varies from 24.3% in Utah to a high of 38.3% in Mississippi.³ While cesarean is often a necessary procedure in cases of fetal intolerance of labor, fetal malpresentation, or labor arrest, this surgery impacts future pregnancy, increasing the risks of the need for repeat surgical deliveries, uterine rupture during attempted labor, and placenta accreta spectrum disorders, the latter of which experts now recognize start as cesarean scar ectopic pregnancies.

Following any full-thickness myometrial surgery, just as with any other muscle, the transected myometrial fibers never truly heal together as they once were. Rather, the reapproximated ends are joined by a line of fibrosis, which may partially dehisce, and lack the usual thickness and elasticity of uncompromised myometrium. This scar sometimes retains some structural integrity, but often results in a “niche” or hollowed-out area within the muscle,^4,5 where an embryo may implant.

Cesarean scar ectopic pregnancy (CSEP) is easiest to detect sonographically early in the early first trimester, when it can be recognized as a low implantation that is offset more anteriorly than normal, often leaving the endometrial cavity and cervical canal empty.⁶ Additional criteria for the diagnosis of  CSEP include thin or undetectable myometrium between the placenta and bladder and unusually increased vascularity between the placenta/sac and bladder or internal cervical os. 

Some experts have proposed simple-to-perform measurements, such as the “crossover sign,” whereby a line is drawn in the sagittal plane connecting the internal os with the fundus by drawing a line along the endometrial canal.⁷ When the superior-inferior diameter of the gestational sac is measured perpendicularly to this line, the relationship of the gestational sac to the endometrial line can be determined.⁶ In CSEPs in which a majority of the gestational sac is closer to the anterior wall (crosses over the endometrial line), there is a higher rate of severe forms of placenta accreta spectrum and risk for rupture when CSEPs are managed conservatively.^7,8

As pregnancy progresses, the gestational sac may grow into the endometrial cavity, making the diagnosis more difficult. Treatment is also more difficult as pregnancy progresses, whereby medical or mechanical management options, such as use of a double balloon catheter are less effective, and surgical resection or combination therapy may be required. The risk of complications including uterine rupture, massive bleeding, and need for emergent hysterectomy that approaches 53% of reported cases are reasons why expectant management is not recommended, especially as early treatment has efficacy rates that are reported between 65% to more than 99% with low complication rates, and with a high likelihood of fertility preservation.  

Early recognition and referral are paramount and provide one more example of how ultrasound can and does save lives.

References:

1. Calì G, Timor-Tritsch IE, Palacios-Jaraquemada J, et al. Outcome of Cesarean scar pregnancy managed expectantly: systematic review and meta-analysis. Ultrasound Obstet Gynecol. 2018; 51:169–175. doi:10.1002/uog.17568

2. Timor-Tritsch IE, Monteagudo A. Unforeseen consequences of the increasing rate of cesarean deliveries: early placenta accreta and cesarean scar pregnancy. A review. Am J Obstet Gynecol 2012; 207:14–29.

3. Centers for Disease Control. Stats of the States – Cesarean Delivery Rates. https://www.cdc.gov/nchs/pressroom/sosmap/cesarean_births/cesareans.htm. Accessed March 30, 2024.

4. Einerson BD, Comstock J, Silver RM, Branch DW, Woodward PJ, Kennedy A. Placenta accreta spectrum disorder: uterine dehiscence, not placental invasion. Obstet Gynecol 2020; 135:1104–1111. doi:10.1097/AOG.0000000000003793.

5. Jauniaux E, Jurkovic D, Hussein AM, Burton GJ. New insights into the etiopathology of placenta accreta spectrum. Am J Obstet Gynecol 2022; 227:384–391. doi:10.1016/j.ajog.2022.02.038.

6. Timor-Tritsch IE, Monteagudo A, Calì G, D’Antonio F, Kaelin Agten A. Cesarean scar pregnancy: diagnosis and pathogenesis. Obstet Gynecol Clin North Am 2019; 46:797–811. doi:10.1016/j.ogc.2019.07.009.

7. Cali G, Forlani F, Timor-Tritsch IE, Palacios-Jaraquemada J, Minneci G, D’Antonio F. Natural history of Cesarean scar pregnancy on prenatal ultrasound: the crossover sign. Ultrasound Obstet Gynecol 2017; 50:100–104. doi:10.1002/uog.16216.

8. Calì G, Calagna G, Khalil A, Polito S, Labate F, Cucinella G, D’Antonio F. First trimester prediction of uterine rupture in cesarean scar pregnancy [published online ahead of print April 20, 2022]. Am J Obstet Gynecol. doi: 10.1016/j.ajog.2022.04.026.

9. Miller R, Gyamfi-Bannerman C. Society for Maternal-Fetal Medicine consult series #63: cesarean scar ectopic pregnancy. Am J Obstet Gynecol 2002 Sep: 227(3):B9–B20. doi: https://doi.org/10.1016/j.ajog.2022.06.024.