The treatment of breast cancer (BC) diagnosed during pregnancy is a challenging situation for the patient, family and healthcare providers. Here we describe the case of a 35-year-old woman diagnosed with a triple-negative breast cancer relapse during pregnancy. She previously underwent neoadjuvant chemotherapy without any response, subsequent left skin sparing mastectomy plus axillary node dissection and radiation therapy to the chest wall and supraclavicular lymph nodes. Two years later, during her first single pregnancy, the patient presented a subclavian vein thrombosis and a BC relapse to locoregional lymph nodes. At 24 weeks of gestation, a first line treatment with weekly paclitaxel and carboplatin was started. Considering the disease progression after two complete cycles of chemotherapy, the patient had an elective caesarean section at 32+6 weeks. A full-body CT-scan and a PET-scan after the delivery showed a massive neoplastic thrombosis involving the left jugular, brachiocephalic and internal mammary vein, as well the superior vena cava and the right atrium. Few data are available on platinum-based chemotherapy during pregnancy in BC patients. Nevertheless, the choice of therapy was conditioned by the previous absence of response to anthracycline and taxane. In case of BC diagnosis during pregnancy, a multidisciplinary management as in the case described is recommended to increase the chance of survival both for the patients and their babies.