BACKGROUND: Preterm birth is a global problem in Perinatal and infant Health. Currently is gaining a growing attention. Rates of preterm birth have increased in most countries, producing a dramatic impact on public health. Factors of diverse nature have been associated to these trends.In Chile, preterm birth has increased since 90. Simultaneously, the advanced demographic transition has modified the characteristics of woman population related to maternity.The principal objective of this study is to analyze some sociodemographic characteristics of the maternal population over time, and their possible association to rates of preterm birth. The second aim is to identify groups of mothers at high risk of having a preterm child. METHODS: This population-based study examined all liveborn singletons in Chile from 1991 to 2008; divided in three periods. Preterm birth rates were measured as % births <37 weeks of gestation.Logistic regression assessed the risk of preterm birth associated with mother's age, parity, and marital status, expressed as crude and adjusted odds ratios. RESULTS: Over time, rates of preterm birth increased in overall population, especially during the third period (2001--2008). In the same time, characteristics of maternal population changed: significant increase of extreme reproductive ages, significant decrease in parity and increase in mothers living without a partner.Risk of preterm birth remained higher in groups of mothers: < 18 and >38 years of age; without a partner; primiparas and grandmultiparas. However, global increase in preterm birth was not explained by the modification of socio demographics characteristics of maternal population. CONCLUSIONS: Some socio demographic characteristics remained associated with preterm birth over time. These associations allowed identifying five groups of mothers at higher risk to have a preterm child in the population.Increase in overall preterm birth affected all women, even those considered at "low sociodemographic risk" and the contribution of more recent period (2001--2008) to this increase is greater.Then, studied factors couldn't explain the increase in preterm birth. Further research will have to consider other factors affecting maternal population that could explain the observed trend of preterm birth.