Effect of older maternal age on the risk of spontaneous preterm labor: a population-based study.

To determine if older maternal age (35 years and older) at first birth was an independent risk factor for spontaneous preterm labor, we conducted a retrospective population-based cohort study. Using provincial perinatal data, we developed separate risk models for low- and high-risk women using multivariate logistic regression. We found that older maternal age exerted a direct and independent effect on spontaneous preterm labor for both nulliparous women with no preexisting chronic illnesses or pregnancy complications (low-risk) and nulliparous women who did not have any preexisting chronic illnesses, but developed one or more pregnancy complications (high-risk).