How is a scar site pregnancy diagnosed?

Diagnosis of Cesarean Scar Pregnancy

Cesarean scar pregnancy (CSP) is primarily diagnosed using transvaginal ultrasound, which should identify a gestational sac implanted in the uterine window at the site of a previous cesarean scar. 1, 2

Primary Diagnostic Modality: Transvaginal Ultrasound

• Transvaginal ultrasound is the first-line diagnostic tool for identifying CSP, with increasing importance as cesarean delivery rates rise worldwide 2, 3
• Key ultrasound findings include:
  • Gestational sac embedded in the uterine window at the site of a cesarean scar 1
  • Empty uterine cavity and cervical canal 4
  • Thin or absent myometrial layer between the gestational sac and bladder 5
  • Absence of a "sliding sign" between the uterus and gestational sac (the gestational sac doesn't move independently with gentle probe pressure) 6

Ultrasound Diagnostic Criteria

• Visualization of the gestational sac in the lower uterine segment at the site of the cesarean scar 1, 4
• Three-dimensional coronal reconstructed images may help confirm the diagnosis 1
• Sign of myometrium partially surrounding the gestational sac 1
• Thin band of myometrium between the endometrium and gestational sac 1

Role of MRI in Diagnosis

• MRI may be used as a secondary diagnostic tool when ultrasound findings are inconclusive 1, 7
• MRI features associated with CSP include:
  • Dark intraplacental bands on T2-weighted imaging 1
  • Abnormal bulging of the placenta or uterus 1
  • Disruption of the zone between the uterus and the placenta 1
  • Abnormal or disorganized placental blood vessels 1
• MRI has comparable diagnostic accuracy to ultrasound (sensitivity 94.4%, specificity 84.0%) but is more expensive and less widely available 1

Diagnostic Challenges and Pitfalls

• CSP can be misdiagnosed as:
  • Cervical pregnancy 1
  • Spontaneous abortion in progress 6
  • Early intrauterine pregnancy 4
• Diagnostic accuracy depends significantly on the examiner's experience and expertise 1
• Interobserver variation in ultrasound interpretation is substantial, with sensitivities ranging from 53.4% to 74.4% and specificities from 70.8% to 94.8% 1

Clinical Risk Factors to Consider

• History of previous cesarean section(s) - risk increases with multiple cesarean deliveries 8
• Previous placental pathology 8
• Previous ectopic pregnancy 8
• Previous cesarean breech delivery 8

Recommendations for Optimal Diagnosis

• Refer women with clinical risk factors to centers with experience and expertise in imaging and diagnosis of CSP 1
• Maintain high clinical suspicion in women with previous cesarean sections who present with menstrual abnormalities or early pregnancy symptoms 3
• Early diagnosis is critical as untreated CSP can lead to serious complications including hemorrhage, loss of fertility, and maternal death 3, 4
• If CSP is diagnosed, patients should be counseled about the high risk (approaching 100%) of developing placenta accreta spectrum if the pregnancy continues 1, 2

Follow-up After Diagnosis

• Serial serum human chorionic gonadotropin (hCG) determinations should be obtained to monitor treatment response 9
• Three-dimensional ultrasound assessment of gestational sac volume and vascularization can be used to monitor resolution after treatment 9