Separation sign: novel ultrasound sign for ruling out diagnosis of placenta accreta spectrum

OBJECTIVE: To assess the performance of the ‘separation sign’ as a predictor of normal placental separation in a large cohort of women at risk for placenta accreta spectrum (PAS) and in a high‐risk subgroup with placenta previa or anterior low‐lying placenta and at least one previous Cesarean delivery. METHODS: This was a prospective study of women at risk for PAS referred to a specialist clinic at between 22 and 38 weeks' gestation. All women underwent ultrasound assessment for the presence of the separation sign, which detects the difference in elasticity between the myometrium and the placenta, characterized by different rates of rebound after an ultrasound probe is used to apply pressure over the uteroplacental interface. When the sign is positive, the placenta appears to move relative to the myometrium, leading to the appearance or enhancement of the clear zone. The predictive performance of the separation sign for normal spontaneous placental separation at delivery was assessed. RESULTS: Of the 194 included women, 163 had a positive separation sign, all of whom went on to have normal placental separation at delivery. Of the 24 women with a negative separation sign, three (12.5%) had normal placental separation and 21 (87.5%) were diagnosed with PAS. This yielded a sensitivity of 98.2% (95% CI, 94.8–99.6%) and specificity of 100% (95% CI, 83.9–100%). In the high‐risk cohort (n = 35), a positive separation sign remained a reliable predictor of normal placental separation, with a positive predictive value of 100%, sensitivity of 88.9% (95% CI, 65.3–98.6%) and specificity of 100% (95% CI, 80.5–100%). CONCLUSIONS: The separation sign could be a useful tool in women considered to be at risk for PAS, as it can facilitate the prediction of normal placental separation at delivery. This may prevent overtreatment, the associated iatrogenic morbidity and unnecessary allocation of clinical resources. © 2022 The Authors. Ultrasound in Obstetrics & Gynecology published by John Wiley & Sons Ltd on behalf of International Society of Ultrasound in Obstetrics and Gynecology.