Objective:To investigate the diagnostic efficacy of ultrasound in pregnancy associated breast cancer (PABC) under different breast ultrasound background echotextures.Methods:The ultrasonic images of 269 female patients with breast diseases who underwent breast surgery in Fudan University Shanghai Cancer Center from January 2016 to September 2023 and were pregnant or within one year postpartum at the time of onset were retrospectively reviewed. Breast ultrasound background echotextures were classified according to two criteria: the first classification was homogeneous-fat, homogeneous-fibroglandular, and heterogeneous; the other classification was hypoechoic dominated and hyperechoic dominated. The comparison of the diagnostic value of ultrasound in PABC under different backgrounds was conducted by the receiver operating characteristic(ROC) curves.Results:Among 269 patients, 67 patients(24.91%)were during pregnancy and 202 patients(75.09%) were within one year postpartum. Pathologically, 47 patients (17.47%) were confirmed as benign, 222 patients (82.53%) were malignant. According to the first classification, 138 patients were homogeneous-fibroglandular and 131 patients were heterogeneous, with the diagnostic sensitivity of ultrasound in PABC were 88.70% and 59.81% respectively, and the specificity were 91.30% and 83.33% respectively, the areas under the ROC curves were 0.940 and 0.826 respectively ( P=0.022). According to the second classification, 119 were hypoechoic dominated and 150 patients were hyperechoic dominated, the sensitivity were 60.21% and 85.27% respectively, the specificity 84.62% and 90.48% respectively, the areas under the ROC curves were 0.826 and 0.925 ( P=0.042). Conclusions:The heterogeneous background echotextures of the breast may cause decrease of the diagnostic efficiency of ultrasound in PABC, and hypoechoic dominated background was more unfavorable for the diagnosis of PABC compared to the hyperechoic dominated background.