Trastuzumab increases survival amongst women with human epidermal growth factor receptor (HER)‐2 receptor positive metastatic breast cancer, but maternal and fetal risks are associated with advanced disease and its treatment in pregnancy. We present a case of a primigravid with HER‐2 positive metastatic breast cancer who received trastuzumab throughout pregnancy. She presented with cerebral metastases, requiring surgical decompression and resection. Reversible oligohydramnios developed during pregnancy. Fetal safety data on trastuzumab in pregnancy is limited, but case reports suggest a recurring pattern of (mostly reversible) oligohydramnios.