Ectopic pregnancy is the leading cause of pregnancy-related first-trimester death in the United States, occurring in approximately 2% of pregnancies [1]. They are most commonly located in the fallopian tube, with 75–80% in the ampullary portion, 10% in the isthmic portion, 5% in the fimbrial end, 2–4% in the interstitial end, also known as cornual ectopic, and 0.5% in the ovary. Abdominal, cervical, and cesarean section scar ectopic pregnancies are rare [2].
Cesarean scar ectopic pregnancy is an unusual type of ectopic where the embryo implants in the myometrium of a previous cesarean scar. The exact incidence is unknown. It has been estimated to range from 1:1800 to 1:2216 pregnancies based on one study following a single center and one case series [3, 4].
It is hypothesized that the conceptus invades into the myometrium through a microscopic defect in the scar. This defect is secondary to poor vascularization of the lower uterine segment with subsequent fibrosis and incomplete healing. As such, the gestational sac is completely surrounded by myometrium and scar tissue and is completely separate from the endometrial cavity [3].
Patients may present with vaginal bleeding, abdominal pain, or hemodynamic instability, but it may also be an incidental finding. Rotas et al. [5] found that 36.8% of patients in their case series were asymptomatic at initial presentation.
Ultrasound is the primary diagnostic modality. Rotas et al. [5] report in their case series that endovaginal ultrasound correctly diagnosed 94 of 111 cases, a sensitivity of 84.6% (95% CI 0.763–0.905). The remaining 17 cases were incorrectly diagnosed as cervical pregnancies or incomplete abortions [5].
Vial et al. [6] proposed the following ultrasound criteria, which have generally been accepted as diagnostic.
In addition, Godin et al. [7] describe an absence of healthy myometrium between the bladder and the sac. The thickness of the myometrium between the gestational sac and the bladder has been reported to be less than 5 mm in two-thirds of cases [8]. Jurkovic et al. [3] also describe the negative ‘sliding organ sign’, defined as the inability to displace the gestational sac from its position at the level of the internal os using gentle pressure applied by the endovaginal probe. Maymon et al. [9] support the use of transabdominal scanning with a full bladder as an adjunct to appreciate a ‘panoramic view’ of the uterus and to acquire an accurate measurement of the distance between the gestational sac and the bladder.
Color Doppler may enhance the diagnostic ability of endovaginal ultrasound by demonstrating peritrophoblastic perfusion surrounding the gestational sac (Fig. 3). Spectral Doppler should demonstrate high velocity (peak velocity >20 cm/sec), low impedance (pulsatility index <1) waveforms [3].
Because this is such a rare condition, there is no standardized approach to the treatment. The medical literature has reported the use of systemic methotrexate, local injection of embryocides, surgical sac aspiration, hysteroscopic evacuation, laparoscopic removal, open surgical treatment, and hysterectomy [10]. Most authors agree that expectant management is not appropriate given the significant risk of uterine rupture [9]. The literature also consistently reports that dilation and curettage are inadequate because the trophoblastic tissue is actually located outside the uterine cavity and unreachable. Such attempts can potentially rupture the uterine scar with devastating consequences [5].
The major differential diagnoses to consider are cervicoisthmic pregnancy and spontaneous abortion in progress. Distinguishing these entities from a cesarean scar ectopic can be difficult, and as the pregnancy progresses, the distinction between cesarean scar ectopic, cervical pregnancy, and low intrauterine pregnancy becomes even more difficult [3]. In a cervicoisthmic pregnancy, there should be a layer of healthy myometrium between the bladder and the gestational sac [7]. In a spontaneous abortion in progress, the gestational sac should be seen in the cervical canal, and color Doppler should demonstrate an avascular sac, unlike a well-perfused cesarean scar ectopic [3].
