The production of biocompatible implants by 3D printing as well as the development of new biomaterials suitable for 3D printing processes is crucial for novel medical applications. Until now, quality monitoring of products manufactured by medical 3D printing has been based mainly on empirical data from the printing process and post-process testing. The lack of in-line quality assurance for additively manufactured parts is a critical technological barrier as it prevents the wider use of additive technologies, especially for high-value applications such as medical products. In the future, in-line quality assurance would be of particular importance for printing personalized medical devices. In this contribution, the use of optical coherence tomography (OCT) for quality sizing of 3D printing processes is presented. The development and integration of an optical measurement system for 3D printer systems, such as an extrusion-based bioprinter and a selective laser sintering-based printer, is demonstrated. This included the system design and setup of OCT measurement heads specifically for both printer types, as well as the evaluation of the recorded in-line and off-line OCT data regarding the influence of different printing parameters and materials. The newly developed system design strongly depends on the process parameters, the available space in the 3D printer, and the printing process itself.