Context.— Canada and the United States have
reported a recent increase in the incidence of preterm birth, but the
reasons for this increase are unknown.
Objective.— To assess secular trends in preterm birth and
its potential determinants.
Design.— Hospital-based cohort study.
Setting.— Canadian tertiary care university teaching
Participants.— A total of 65,574 nonreferred live
births and stillbirths.
Main Outcome Measures.— Changes in occurrence of
preterm birth, before and after adjustment for changes in method of
gestational age assessment, obstetric intervention, registration of
births weighing less than 500 g, and sociodemographic, behavioral, and
Results.— A crude secular increase in preterm births was
seen for births less than 37, 34, and 32 completed weeks using 3
alternative gestational age estimation methods. Based on an algorithm
incorporating both menstrual and early ultrasound gestational age
estimates, rates increased from 6.6% to 9.8% for births at less than
37 weeks' gestation, 1.7% to 2.3% at less than 34 weeks, and 1.0%
to 1.2% at less than 32 weeks. Exclusion of births weighing less than
500 g and those with induction or preterm cesarean delivery without
labor before each of the corresponding gestational age cutoffs
eliminated the secular trends for births before 34 and 32 weeks and
attenuated the trend for births before 37 weeks. Nearly half of the
remaining trend for births before 37 weeks was accounted for by the
increasing use of early ultrasound dating. The residual trend was
eliminated after controlling for secular increases in unmarried status
and the proportion of women aged 35 years or older. These factors,
combined with a decrease in alcohol consumption and increases in
histological chorioamnionitis and cocaine use, appear to have
counteracted a reduction in preterm birth since the mid-1980s that
otherwise would have been observed.
Conclusions.— This hospital's increase in preterm births
since 1978 parallels increases reported in population-based national
studies from the United States and Canada. This trend appears largely
attributable to the increasing use of early ultrasound dating, preterm
induction and preterm cesarean delivery without labor, and changes in
sociodemographic and behavioral