SECULAR TRENDS IN PRETERM BIRTH - A HOSPITAL-BASED COHORT STUDY

Context.-Canada and the United States have reported a recent increase in the incidence of preterm birth, but the reasons for this increase a re unknown. Objective.-To assess secular trends in preterm birth and i ts potential determinants. Design.-Hospital-based cohort study. Settin g.-Canadian tertiary care university teaching hospital, 1978-1996. Par ticipants.-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 assessme nt, obstetric intervention, registration of births weighing less than 500 g, and sociodemographic, behavioral, and clinical determinants. Re sults.-A crude secular increase in preterm births was seen for births less than 37, 34, and 32 completed weeks using 3 alternative gestation al age estimation methods, Based on an algorithm incorporating both me nstrual and early ultrasound gestational age estimates, rates increase d 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 corresp onding gestational age cutoffs eliminated the secular trends for birth s 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. Th e residual trend was eliminated after controlling for secular increase s in unmarried status and the proportion of women aged 35 years or old er. 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 th at otherwise would have been observed. Conclusions.-This hospital's in crease in preterm births since 1978 parallels increases reported in po pulation-based national studies from the United States and Canada. Thi s trend appears largely attributable to the increasing use of early ul trasound dating, preterm induction and preterm cesarean delivery witho ut labor, and changes in sociodemographic and behavioral factors.