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Optimal management of placenta accreta spectrum (PAS) requires antenatal diagnosis. We sought to evaluate the sensitivity of ultrasound findings suggestive of PAS in detecting posterior PAS. Cohort study of patients with posterior placentation and pathology-confirmed PAS from 2011 to 2020 at a tertiary center. Patients were excluded if ultrasound images were unavailable. Ultrasounds were reviewed for presence of lacunae, hypervascularity, myometrial thinning, loss of the hypoechoic zone, bridging vessels, abnormal uterine serosa–bladder interface, placental bulge, placental extension into/beyond the myometrium, and an exophytic mass. Risk factors, postpartum outcomes, and ultrasound findings were compared by antepartum suspicion for PAS. Sensitivity was calculated for each ultrasound finding. Thirty-three patients were included. PAS was not suspected antenatally in 70 % (23/33). Patients with unsuspected PAS were more likely to be non-Hispanic, have in vitro fertilization, no prior Cesarean deliveries, no placenta previa, and delivered later in gestation. Depth of invasion and estimated blood loss were less for unsuspected PAS, but there was no difference in hysterectomy between groups. Ultrasound findings were less frequently seen in those who were not suspected antenatally: lacunae 17.4 vs. 100 % (p<0.001), hypervascularity 8.7 vs. 80 % (p<0.001), myometrial thinning 4.4 vs. 70 % (p<0.001), and placental bridging vessels 0 vs. 60 % (p<0.001). There was poor sensitivity (0–42.4 %) for all findings. Posterior PAS is less likely to be detected antenatally due to a lower sensitivity of typical ultrasound findings in the setting of a posterior placenta. Further studies are needed to better identify reliable markers of posterior PAS. [ABSTRACT FROM AUTHOR]