Ulcerative colitis (UC) classically presents with abdominal pain, hematochezia, or diarrhea. However, it can present atypically in pediatric and pregnant patients, posing a diagnostic challenge. A healthy, 16-year-old primigravida presented at 18 weeks and six days of gestation with sudden-onset altered mental status and severe anemia. Hematochezia began about 12 hours after admission. She underwent extensive workup, leading to an endoscopic and histopathologic diagnosis of UC, and achieved prenatal remission with high-dose steroids and infliximab. Her pregnancy, however, was complicated by severe preeclampsia, and her child's post-delivery course was medically complex from an unrelated etiology. Pregnancy-onset inflammatory bowel disease (IBD) in the pediatric population is an uncommon but important consideration. Early diagnosis, treatment, and counseling are vital to achieve results comparable to those of patients without IBD.