The etiologies of undifferentiated fever in pregnant women have not been studied thoroughly. Because of its non-specific presentation but severe prognosis, listeriosis is often suspected in this setting, but in most cases not confirmed. We studied the causes of undifferentiated fever in pregnant women who received preemptive listeriosis treatment. We conducted from November 1, 2011, to June 30, 2013, a prospective multicentric observational cohort study of pregnant women referred to obstetrical wards with undifferentiated fever and who received listeriosis preemptive treatment. Clinical and biological features, treatment, outcome, and final diagnosis were collected. We enrolled 103 febrile pregnant women. A cause was identified in 77/103 (75%): viral infection in 52/103 (50%, influenza in 21 (20%)), bacterial infection in 22 (21%, including 16 pyelonephritis (16%) and 3 pneumonias (3%)), and TORCH infection in 3 (3%, varicella, toxoplasmosis, and cytomegalovirus primo-infections, n=1, each). Viral infections collected during influenza outbreaks (December-March) accounted for 43/57 (75%) cases. Two fetal losses were reported in the context of febrile pneumonia. Final diagnoses required adapting medical care in 46/77 (60%) of cases, for bacterial, influenza, or TORCH infections. A large array of benign to potentially severe infections manifests as acute undifferentiated fever in pregnant women, requiring careful repeated evaluation.