Inflammatory bowel diseases, Crohn's disease, and ulcerative colitis have an unpredictable course during and after pregnancy (1). There is a great deal of interest in treating moderate to severe active inflammatory bowel disease with anti-tumor necrosis factor (anti-TNF) biologics in pregnant women (2). We lack definitive information about the effects of these agents on the development of the immune system of the human fetus and the newborn baby. Anti-TNF agents fall within US Food and Drug Administration's (FDA) category B regarding fetal risk, indicating that no adequate and well-controlled studies have been conducted in pregnant or nursing women. Here, we review animal studies (of both mice and nonhuman primates) that examine the role of TNF and its inhibitors in the normal development of the immune system.