Background <p>Anatomical and functional changes in the lactating breast may increase the risk of complications after surgical intervention. We examined postoperative complications in peripartum patients undergoing breast surgery.</p> Patients and Methods <p>This retrospective study included pregnant and postpartum patients undergoing oncologic breast surgery from 1989 to 2022. Primary outcome was 90-day complication rate. Patients &gt; 20&#xa0;weeks gestational age or postpartum were considered to have lactational breast changes. Descriptive statistics and logistic regression analyses were performed.</p> Results <p>Of 164 patients, most had high-grade (<i>n</i> = 104, 66.7%), ductal (141, 89.8%), clinical stage II–III (<i>n</i> = 143, 87.2%) invasive carcinoma, and 59.8% (<i>n</i> = 98) received neoadjuvant chemotherapy (NAC). The post-biopsy complication rate was 0.63% (1/160, seroma). At time of surgery, 63 patients (38.4%) were pregnant and 101 (61.6%) were postpartum; 122 (75.0%) had lactational breast changes. The complication rate after surgery (72% total mastectomy, 28% segmental mastectomy) was 12.2% (95% CI 7.6–18.2). Surgical site infection was the most common (<i>n</i> = 10, 6.1%), followed by fluid collections (<i>n</i> = 7, 4.9%; six seromas, one hematoma), and mastectomy flap necrosis (<i>n</i> = 2, 1.2%). No milk fistulae were observed. Postoperative complications were not associated with lactational breast changes in logistic regression models controlling for either extent of breast surgery or NAC receipt (<i>p</i> &gt; 0.05).</p> Conclusions <p>In the largest reported cohort of peripartum patients undergoing breast surgery, the postoperative complication rate was low, irrespective of lactational breast changes, and similar to published rates for the general population. These results support timely surgical treatment of pregnant and postpartum patients per standard of care regardless of lactational status.</p>