Objectives. We sought evidence to support the hypothesis that advancing maternal age is potentially causing a rise in preterm birth (PTB) rates in high-income countries. Methods. We assessed maternal age-specific trends in PTB using all singleton live births in Denmark (n = 1 674 308) and Quebec (n = 2 291 253) from 1981 to 2008. We decomposed the country-specific contributions of age-specific PTB rates and maternal age distribution to overall PTB rates over time. Results. PTB rates increased from 4.4% to 5.0% in Denmark and from 5.1% to 6.0% in Quebec. Rates increased the most in women aged 20 to 29 years, whereas rates decreased or remained stable in women aged 35 years and older. The overall increase over time was driven by age-specific PTB rates, although the contribution of younger women was countered by fewer births at this age in both Denmark and Quebec. Conclusions. PTB rates increased among women aged 20 to 29 years, but their contribution to the overall PTB rates was offset by older maternal age over time. Women aged 20 to 29 years should be targeted to reduce PTB rates, as potential for prevention may be greater in this age group.