I’ve never seen anything like this before. Courtesy of the NEJM:

A 24-year-old woman, para 2, who was 36 weeks pregnant, presented with massive diffuse enlargement of both breasts, which had progressed since conception (Panel A). There had been no such enlargement during her earlier pregnancies. Multiple cutaneous ulcerations occurred as a result of the rapid enlargement of the breasts over a period of 4 to 5 months. The breasts were normal on ultrasonography. Fine-needle aspiration revealed hyperplasia of the mammary glands that was consistent with pregnancy (Panel B), with no evidence of fibrosis or cancer. Supportive treatment was given until delivery at 38 weeks of gestation. The patient did not have a response to bromocriptine therapy, and reduction mammaplasty was performed 6 weeks post partum.