Objective: To propose a new classification of preterm births into three groups, by studying a population of preterm infants born before 35 weeks of gestation. To describe this population and the pregnancy complications associated with each group. Patients and Methods: In two areas covered by a perinatal network, all preterm births, live births and stillbirths, which occurred between 22 and 34 completed weeks were recorded over a 21 months period. Each case was classified either in the medically indicated preterm birth (I) group, or in the accepted spontaneous preterm birth (ASp) group or in the non-accepted spontaneous preterm birth (NASp) group. Results: 1060 cases of preterm births were included; among them 981 were live births or ended with per partum infant death. 49% of these births were medically indicated, 32% were ASp and 19% were NASp. The distribution of pregnancy complications and fetal disorders differed between preterm birth groups: ischemic placental diseases were present in 38,2% of medically indicated births; preterm premature rupture of membranes occurred twice more often in I and ASp preterm births than in NASp preterm births. Conclusion: This classification is based on the medical decision; it allows to compare medical practices in given obstetrical situations. It appears to be reproducible and easy to use.