RATIONALE: Hypercoagulability and pregnancy morbidity are hallmarks of the antiphospholipid syndrome (APS). Catastrophic antiphospholipid syndrome (CAPS) is a potentially life-threatening omplication of APS, with widespread acute thrombotic microangiopathy (TMA) that can be precipitated by pregnancy and delivery and result in multiorgan damage. Unrestrained activation of the complement cascade is involved, favoring endothelial activation, tissue factor expression by leukocytes, and platelet aggregation. The complement block, which interrupts this amplification cycle, could prevent CAPS in patients with early TMA who face precipitating events.PATIENT CONCERNS: We present a nulliparous pregnant woman with APS at the 30 week of gestation who has developed thrombocytopenia, intravascular hemolysis, elevated creatinine, proteinuria, and hematuria.DIAGNOSES: These featurs were compatible with the diagnosis of CAPS. Consensually, serum C3 protein levels were rapidly decreasing, reflecting complement consumption.INTERVENTIONS: She was treated with eculizumab, a humanized monoclonal antibody against C5 that prevents the formation of the complement membrane attack complex.OUTCOMES: Laboratory parameters improved and the patient did not develop thrombosis or detectable organ/tissue damage. The patient safely delivered by cesarean section at week 32 of gestation a healthy 1640 g male infant. After 5 days, she received additional eculizumab, with complete resolution of the clinical condition. Low complement activity was detectable in the infant blood for a week after delivery. No infectious complication occurred.LESSONS: Inhibition of the terminal complement activation is safe and might be effective in patients with APS developing early TMA, enabling safe delivery and preventing thrombotic events both in the mother and in the newborn.