Accurate identification of cohort study designs in perinatal research: a practical guide

Study designs are often mischaracterized in the obstetrics literature; in particular, the designation of studies as retrospective (historical) cohorts is frequently in error to describe studies that are prospective cohorts. This is especially true for studies based on electronic health records, which often should be properly considered as prospective cohorts. Epidemiologic study designs were developed in earlier eras of research and healthcare when researchers directly contacted study participants or relied on data from paper medical records. Accordingly, standard epidemiologic study design definitions are difficult to apply to digitized data, which have become common in the modern era of healthcare and computing. In this article, we briefly review the characteristics of the 3 main types of cohort studies. Afterward, we build on existing definitions by proposing several subdesignations of prospective cohort studies that we believe will reduce the confusion in terminology. We provide illustrative examples from obstetrics to concretely demonstrate connections and distinctions among study designs. First, a prospective cohort study can be "active" (participants are deliberately and explicitly enrolled in a prospective research study) or "passive" (participants are followed up in real time for some nonresearch activity, such as clinical care or quality improvement). An active prospective cohort study never stops being a prospective cohort study; however, when reused to answer a new, secondary question, we propose that this should be called a "reused (active) prospective cohort." The de novo cohort study that answered the original question should be considered an "intended (active) prospective cohort." Lastly, when a randomized controlled trial is reused to study some new questions where the randomization variable is not under study, this is also a subtype of a prospective cohort study, a "repurposed randomized controlled trial." The use of more detailed descriptors to describe prospective cohort studies will enable more accurate identification of this study design going forward. It is likely that further refinements will be needed in the future, given the ongoing evolution of how we engage with patients or participants and how data are collected, stored, and linked.