International comparison of common risk factors of preterm birth between the U.S. and Canada, using PRAMS and MES (2005-2006).

Preterm birth (PTB) is a leading cause of newborn deaths and morbidities. The pregnancy risk assessment monitoring system (PRAMS) from the U.S., and the maternity experiences survey (MES) from Canada, which was modeled from PRAMS, were used to examine between-country differences in risk factors of preterm birth. The adjusted risk ratio and population attributable fraction (PAF) were calculated for modifiable and semi-modifiable risk factors of PTB, and all measures were compared between the U.S. and Canada. PTB was defined here as a live singleton birth between 28 and 37 completed weeks gestation (using the clinical gestational age estimate) where the baby was living with the mother at the time of the survey. The PTB risk was 7.6 % (SE = 0.2) in the U.S. and 4.9 % (SE = 0.3) in Canada. The a priori high risk category of factors was almost always more prevalent in the U.S. than Canada, suggesting broad social differences, but individually most of these differences were not associated with PTB. The underlying risk of PTB was generally higher in the U.S. in both the higher risk and referent categories, and the risk ratios for most risk factors were similar between the countries. The primary exception was for recurrence of PTB, where the risk ratio (RR) and PAF were much higher in Canada. We observed between-country differences in both the prevalence of risk factors and the adjusted RR. Further between-country comparisons may lead to important inferences as to the influence of modifiable risk factors contributing to PTB.