The incidence of preterm deliveries decreases in Finland

Objective We examined the trends and risk factors of preterm delivery. Design Register-based retrospective cohort study from Finland. Setting National Medical Birth Register data during 1987-2005. Population The study population consisted of 1 137 515 deliveries, of which 59 025 were preterm (5.2%). Methods We calculated the population attributable risks for using the risk factor prevalence rates in the population. We further calculated odds ratios with 95% CI by multivariate logistic regression to adjust for confounders. Main outcome measures Preterm delivery rate subclassified into moderately preterm (32-36 weeks), very preterm (28-31 weeks) and extremely preterm (less than 28 weeks). Results Preterm delivery rates increased from 5.1% in the late 1980s to 5.4% in the late 1990s but then decreased to 5.2% for 2001-05. The proportion of extremely preterm deliveries decreased substantially by 12% (P < 0.01). The greatest risk factors were multiplicity (OR 13.72, 95% CI 13.26-14.19), followed by elective delivery (OR 1.86, 95% CI 1.82-1.89), primiparity (OR 1.47, 95% CI 1.45-1.50), in vitro fertilisation treatment (OR 1.39, 95% CI 1.31-1.47), maternal smoking (OR 1.31, 95% CI 1.29-1.34) and advanced maternal age (OR 1.02, 95% CI 1.02-1.03 for each additional year of age). Prematurity rates decreased by 1.8% after adjusting for risk variables. Conclusions The rate of preterm delivery has not increased from 1987 to 2005 in Finland, while the risk for extremely preterm delivery has decreased. This finding is in contrast with recent trends in other countries.