Preterm birth is associated with breakdown of maternal-fetal tolerance and remains the leading cause of global mortality in young children. However, the role of self-reactive antibodies has been limited to a handful of pathogenic allo- and autoantibodies linked to pregnancy complications. Here we identify a shared autoreactive signature of pregnancy in pregnant sera (n = 2,719) relative to never-pregnant controls (n = 130), using proteome-wide autoantibody profiling. Across eight human pregnancy cohorts, we define autoreactivities associated with preterm birth by comparing preterm (n = 688) and term (n = 1,505) pregnancies. Within the signature, antibodies to IL1 receptor antagonist (IL1RA) induce resorption, inflammation and vascular malperfusion in pregnant mice. IL1RA localizes to placental endothelial adherens junctions, suggesting a role in vascular integrity. These findings identify a term pregnancy humoral immune program and shared changes preceding preterm delivery, including anti-IL1RA, which may have direct pathogenic consequences.