Historical (retrospective) cohort studies and other epidemiologic study designs in perinatal research.

When investigators describe the study design they employed, the term retrospective cohort commonly is selected and commonly used incorrectly. In a prospective cohort, investigators enroll exposed and unexposed individuals, all of whom are at risk of experiencing the study outcome, and follow them forward in time to observe incident outcomes. In a retrospective (historical) cohort, investigators use preexisting data to identify exposed and unexposed individuals in the past, without regard to outcome status, and trace these individuals forward, up to and possibly including the present, to determine incident outcomes. Both of these designs are cohorts because they identify individuals based on exposure, without regard to outcome; they follow the individuals over time, either in the future (prospective cohort) or in the past (historical cohort), and they assess the incidence, not just the prevalence, of the study outcome. The designation of retrospective cohort is based on the presence and timing of follow up before the onset of research, not on the timing of the analysis with respect to when the data were collected and regardless of the original purpose for which the data were collected. In short, a prospective cohort study remains a prospective cohort study when analyzed for secondary research purposes, even if this research occurs many years after data collection. Because of the complex nature of modern databases and study designs and the inherent ambiguity of the terms retrospective and prospective, modern epidemiologists rarely use these terms as simple descriptors, preferring instead to describe what the study actually did.