Maternal-fetal interventions-such as prenatal fetal myelomeningocele (MMC) repair-are at the forefront of clinical innovation within maternal-fetal medicine, pediatric surgery, and neonatology. Many centers determine eligibility for innovative procedures using pre-determined inclusion and exclusion criteria based on seminal studies, for example, the "Management of Myelomeningocele Study" for prenatal MMC repair. What if a person's clinical presentation does not conform to predetermined criteria for maternal-fetal intervention? Does changing criteria on a case-by-case basis (i.e., ad hoc) constitute an innovation in practice and flexible personalized care or transgression of commonly held standards with potential negative consequences? We outline principle-based, bioethically justified answers to these questions using fetal MMC repair as an example. We pay special attention to the historical origins of inclusion and exclusion criteria, risks and benefits to the pregnant person and the fetus, and team dynamics. We include recommendations for maternal-fetal centers facing these questions.