NEW YORK, August 16 (Praxis Press). Preterm birth is defined as birth before 37 gestational weeks. Although infants born at 32 through 36 weeks are more common than very preterm infants (those born before 32 weeks), most studies have focused on very preterm infants because of their high risk of mortality and serious morbidity. In a new study, Kramer and colleagues examined the contribution of mild (birth between 34 and 36 weeks) and moderate (birth between 32 and 33 weeks) preterm birth to infant mortality in the US and Canada using birth cohort data for singleton infants born between 1985 and 1994. Compared with infants born at term, the relative risk of death from all causes was 6.6 in the US and 15.2 in Canada among moderate preterm infants. Among mild preterm infants, the corresponding relative risks were 2.9 and 4.5. There is no clear reason why infant mortality was greater in Canada, but the authors speculated that different demographic characteristics may play a role. Mild and moderate preterm births have a substantial impact on infant mortality.