Background Typically, inflammatory bowel disease (IBD) patients are in their reproductive years, raising questions about safely using antitumour necrosis factor antibodies like infliximab (IFX) during pregnancy. IgG antibodies naturally cross the placenta, especially during the last trimester. To prevent foetal intra-uterine exposure, stopping IFX treatment at gestational week 30 is recommended. However, whether this limits intra-uterine and early postnatal IFX exposure is unestablished. Aim To determine the intra-uterine exposure to IFX following maternal treatment with IFX. Methods Four pregnant IBD patients intentionally continued IFX during pregnancy. IFX levels were assessed in newborns' cord blood and the mothers' peripheral blood at delivery. The children's development during the first 3-6 months, infections, vaccine reactions and antibody responses to vaccinations against Haemophilus influenzae type b and Pneumococcus were assessed. Results The patients stopped IFX therapy at gestational week 21, 26, 26 and 30, respectively. In three infants, therapeutic IFX levels were present in cord blood at levels of 5.5-13.7 mug/mL and were two- to three-fold higher than in the peripheral blood of their mothers. During the 3- to 6-month follow-up, the children developed normally without signs of infections or allergic reactions, and had normal antibody titres after routine childhood vaccinations. Conclusion The use of IFX until gestational week 30 leads to foetal intra-uterine exposure to IFX at levels that exceed those in the mothers' peripheral blood. Although no short-term complications were detected, the high IFX levels observed in newborns raise concerns about unknown effects of IFX on the developing immune system.