Temporal trends in preterm birth rate over the last 30 years in Sweden: a population-based study

ObjectivesTo determine temporal trends in the rates of preterm birth and its sub-types in Sweden and assess the contribution of known-risk factors. DesignA population-based register study. SettingSweden. Participants (Instead of patients or subjects)3,264,146 pregnancies registered in the Swedish Medical Birth Registry with information on pregnancy duration and onset of labour (1991 - 2021). Main outcome measuresThe primary outcomes were the overall, spontaneous and iatrogenic preterm birth rates between 1991 - 2021, stratified on singleton and multiple births, as well as for extremely preterm (<28 weeks, <196 days), very preterm (28-31 weeks, 196 - 224 days), moderately preterm (32 - 33 weeks, 224 - 238 days), and late preterm (34 - 36 weeks, 238 - 259 days) births. Using logistic regression models, we investigated whether maternal age at conception, use of artificial reproductive technologies, smoking, parity, and maternal continent of birth were associated with the observed trends. ResultsThe overall preterm birth rate was stable between 1991 - 2005 at 5.50% (95% CI: 5.37%, 5.63% in 1991) but decreased thereafter to 4.78% (95% CI: 4.66%, 4.91%) in 2021, a finding confined to spontaneous preterm births. The largest decline was observed in late preterm births, from 3.92% (95% CI: 3.80%, 4.05%) in 2005 to 3.52% (95% CI: 3.41%, 3.63%) in 2021. Moderately preterm birth also declined (0.70%, 95% CI: 0.65%, 0.76% in 2005 to 0.53%; 95% CI: 0.49%, 0.58% in 2021), whereas very-extremely preterm birth did not. Decreased spontaneous preterm birth rates were observed in women born in European, Asian and African countries, with largest decline observed in the latter (rate in 1991 = 2.65%, 95% CI: 1.74%, 3.86%; rate in 2021 = 1.72%, 95% CI: 1.42%, 2.07%). Adjusting for maternal and obstetric risk factors didnt alter these associations. ConclusionsWhile rates of preterm birth have been stable or increased globally, they have decreased in Sweden from 2006 - 2021, despite the lack of any nation-wide preventive strategy during this period. Understanding the reasons for this decline will provide useful strategies to make the decline a rule, rather than an exception.