A 36-year-old Japanese woman with intestinal Behcet's disease was admitted to our hospital due to a recurrent ileocecal ulcer. Because infliximab (IFX) showed secondary failure, IFX was switched to adalimumab (ADA). After the third injection of ADA, she was unexpectedly 4-weeks pregnant. ADA was continued until 20 gestational weeks. Remission of the disease activity was maintained during pregnancy, and the birth was uneventful. The ileocecal ulcer disappeared after her delivery. ADA was detected in the umbilical blood after 119 days from the last infusion. The placental transition and timing of neonatal vaccination should be considered in cases of pregnancy with TNF antibody therapy.