Objective Spontaneous preterm birth (SPTB) is a complex condition that is likely a final common pathway with multiple possible causes. We hypothesized that a comprehensive classification system appropriately could group women with similar STPB causes and could provide an explanation, at least in part, for the disparities in SPTB that are associated with race and gestational age at delivery. Study Design This was a planned analysis of a multicenter, prospective study of singleton SPTBs. Women with SPTB at <34 weeks' gestation were included. We defined 9 potential SPTB phenotypes based on clinical data: infection/inflammation, maternal stress, decidual hemorrhage, uterine distention, cervical insufficiency, placental dysfunction, premature rupture of the membranes, maternal comorbidities, and familial factors. Each woman's condition was evaluated for each phenotype. Delivery gestational age was compared between those with and without each phenotype. Phenotype profiles were also compared between women with very early (20.0-27.9 weeks' gestation) SPTB vs those with early SPTB (28.0-34.0 weeks' gestation) and between African American and white women. Statistical analysis was by t test and χ2 test, as appropriate. Results The phenotyping tool was applied to 1025 women with SPTBs who delivered at a mean 30.0 ± 3.2 (SD) weeks' gestation. Of these, 800 women (78%) had ≥2 phenotypes. Only 43 women (4.2%) had no phenotypes. The 281 women with early SPTBs were more likely to have infection/inflammation, decidual hemorrhage, and cervical insufficiency phenotypes (all P≤.001). African American women had more maternal stress and cervical insufficiency but less decidual hemorrhage and placental dysfunction compared with white women (all P <.05). Gestational age at delivery decreased as the number of phenotypes that were present increased. Conclusion Precise SPTB phenotyping classifies women with SPTBs and identifies specific differences between very early and early SPTB and between African American and white women.