Background and Objectives: The breast undergoes extensive physiologic changes during pregnancy/lactation. We aimed to review the ultrasound (US) imaging characteristics of breast lesions during pregnancy/lactation and to demonstrate ultrasonography as an excellent imaging modality in this patient population. Materials and Methods: We performed a retrospective study involving 195 patients with 206 pathologically confirmed breast lesions during pregnancy/lactation over the period of January 2010 to December 2018; 51 were diagnosed with breast cancer, including 50 invasive ductal carcinomas and 1 low malignant potential phyllodes tumor, whereas 144 were diagnosed with 155 benign lesions, including 103 mastitis/abscesses, 45 fibroadenomas, 2 intraductal papillomas, 2 sclerosing lesions, and 3 benign phyllodes tumors. The patients' age, lesion diameter, and US characteristics were analyzed. Results: The breast lesions of patients during pregnancy/lactation were compared with those during nonpregnancy/lactation. Patients with breast cancer or fibroadenomas during pregnancy/lactation were younger than those during nonpregnancy/lactation. The average lesion diameter was significantly higher among pregnant/lactating patients compared with controls of childbearing age for fibroadenomas, but not for malignant lesions. The fibroadenomas and mastitis/abscesses during pregnancy/lactation usually have higher BI-RADS categories than those during nonpregnancy/lactation. Conclusions: The imaging features of breast cancer during pregnancy/lactation did not differ much from those of nonpregnancy/lactation; however, some benign lesions had suspicious sonographic features, and US-guided core biopsies were necessary for differentiating benign from malignant lesions.