Although severe flare of ulcerative colitis (UC) is uncommon, it significantly increases the risk of preterm delivery, low birth weight and other adverse fetal outcomes. It is critical to optimize aggressive medical treatment with both mother and fetal health. Here, we present a case of a 30-year-old woman with a severe flare of UC at the 16th gestational week. The diagnosis of extensive UC was established 8 years ago. From the time she was diagnosed, she had 5 moderate flares successfully treated with oral and topical mesalamine. The relapses of disease occurred due to poor adherence to maintenance therapy. The patient had a positive family history for UC and thrombophilia (factor V Leiden mutations). At the time of admission, she presented with 8-10 bloody diarrheas and moderate abdominal pain. She was afebrile with increased heart rate (96/min). Laboratory studies showed elevated C-reactive protein (CRP, 42 mg/l), fecal-calprotectin (7,223 μg/g), and anemia (hemoglobin 10.4 g/dl). Clostridium difficile and CMV infection were excluded. Intensive treatment with systemic steroids and low-molecular weight heparin was started. Three days later, no response to the therapy was observed (8 bloody stools, CRP 40 mg/l). According to emergency symptoms, rescue therapy with infliximab (IFX; 5 mg/kg standard induction protocol) was administered a week later. A partial clinical and laboratory response was achieved after the second dose of IFX (4 stools/day, CRP 12.2 mg/l and FCP 1,078 μg/g). The patient received the third and least doses of IFX at the 23rd gestational week. She continued on corticosteroids and mesalamine with chronically active moderate disease. IFX trough levels before the third dose were 20.6 μg/ml; antibodies to IFX were negative. The patient delivered trans-vaginally a healthy girl on the 36th gestational week (the newborn weight: 3,150 kg, APGAR score 9). No live vaccines were administrated to the newborn until 6 months of age.