How 37 Weeks Came to Define Preterm Birth and Why It Matters Today—Making of a Threshold

Importance Preterm birth , defined as birth before 37 completed weeks of gestation, affects over 13 million births annually worldwide and remains a leading cause of neonatal morbidity and mortality. Despite the universal adoption of the 37-week threshold to define preterm birth, its historical origin and rationale remain unclear. Observations This review traces the evolution of the concepts of fetal maturity and gestational timing from ancient medicine to modern obstetrics. The idea of birth occurring too early can be traced to ancient Greek and Chinese medical writings, but the current 37-week threshold emerged in the mid-20th century, when definitions of prematurity shifted from birth weight–based criteria to gestational age. However, the basis for this threshold was not explicitly documented, and no clear evidence that its selection was based on a specific biological, clinical, or epidemiological justification was identified. Advances in obstetric dating and neonatal care have reshaped the clinical meaning of gestational age, with modern research and practice increasingly moving beyond a simple preterm-term dichotomy to the introduction of several subcategories of preterm birth. Growing evidence that infants born at 37 to 38 weeks face considerably higher short- and long-term health risks than those born later further challenges the assumption that 37 weeks represents a biologically sharp boundary. Conclusions and Relevance The 37-week threshold for preterm birth has long been used in epidemiological surveillance, research, and clinical classification, although no clear documentation supporting its selection has been identified, and it may be insufficient for a better understanding of short- and long-term health of the offspring. A more precise assessment of gestational age, together with high-resolution clinical and population data, may support a more refined classification of different preterm birth categories and further advance clinical care, research, and public health.