In recent years, the use of large data sets, such as electronic health records, has increased. These large data sets are often referred to as “Big Data,” which have various definitions. The purpose of this article was to summarize and review the utilization, strengths, and challenges of register data, which means a written record containing regular entries of items or details, and Big Data, especially in maternal nursing, using 4 examples of studies from the Finnish Medical Birth Register data and relate these to other international databases and data sets. Using large health register data is crucial when studying and understanding outcomes of maternity care. This type of data enables comparisons on a population level and can be utilized in research related to maternal health, with important issues and implications for future research and clinical practice. Although there are challenges connected with register data and Big Data, these large data sets offer the opportunity for timely insight into population-based information on relevant research topics in maternal health. Nurse researchers need to understand the possibilities and limitations of using existing register data in maternity research. Maternal child nurse researchers can be leaders of the movement to utilize Big Data to improve global maternal health.