Traditionally, the investigation of preterm birth has relied on diagnostic definitions derived from maternal clinical presentation. However, clinical presentation may be only tangentially related to the underlying etiology of a disease. The utilization of data derived directly from the intrauterine or maternal systemic environment would be invaluable in consideration of the causes of preterm birth. In this issue, Kelly et al. (Am J Epidemiol. 2009;170(2)148-158) contribute to our understanding of the epidemiology of the intrauterine environment by classifying the vascular biology of the maternal-placental interface in cases of preterm delivery. Their histology-based approach observes that vascular conditions may be grouped into 5 constructs with specific relations to maternal and fetal vascular pathology. The frequencies of these constructs vary with regard to delivery indication and gestational age, suggesting that the intrauterine conditions associated with preterm birth are more complicated than originally appreciated. This work is laborious, and replication of the technique will be important. However, these authors have taken a large step toward introducing an "intrauterine" perspective into perinatal epidemiology and into our understanding of the underlying etiologies of preterm birth.