This post contains the Companion Guide for Preterm Birth and Assessment of Gestational Age. This and other companion guides have been created as part of the CEPI-funded SPEAC project. They contain resources and tools specific to events that have a published Brighton Collaboration case definition. The resources include risk factors and background rates for the event as well as a summary of key caveats from the case definition to: 1) guide real time investigations needed to meet the case definition as well as to look for diseases that would exclude the event as a case; and 2) guidelines for analysis and presentation of the event as an adverse event of special interest (AESI). The tools include a detailed case report form that guides the abstraction and interpretation of key data from medical records needed to meet case definition criteria as well as two different abbreviated one page formats for summarizing data on the case definition criteria and algorithms to assign the level of certainty.