Geographical differences in preterm delivery rates in Sweden: A population-based cohort study

Abstract

Introduction: Preterm delivery is a major global public health challenge. The objective of this study was to determine how preterm delivery rates differ in a country with a very high human development index and to explore rural vs urban environmental and socioeconomic factors that may be responsible for this variation.

Material and methods: A population-based study was performed using data from the Swedish Medical Birth Register from 1998 to 2013. Sweden was chosen as a model because of its validated, routinely collected data and availability of individual social data. The total population comprised 1 335 802 singleton births. A multiple linear regression was used to adjust gestational age for known risk factors (maternal smoking, ethnicity, maternal education, maternal age, height, fetal sex, maternal diabetes, maternal hypertension, and parity). A second and a third model were subsequently fitted allowing separate intercepts for each municipality (as fixed or random effects). Adjusted gestational ages were converted to preterm delivery rates and mapped onto maternal residential municipalities. Additionally, the effects of six rural vs urban environmental and socioeconomic factors on gestational age were tested using a simple weighted linear regression.

Results: The study population preterm delivery rate was 4.12%. Marked differences from the overall preterm delivery rate were observed (rate estimates ranged from 1.73% to 6.31%). The statistical significance of this heterogeneity across municipalities was confirmed by a chi-squared test (P < 0.001). Around 20% of the gestational age variance explained by the full model (after adjustment for known variables described above) could be attributed to municipality-level effects. In addition, gestational age was found to be longer in areas with a higher fraction of built-upon land and other urban features.

Conclusions: After adjusting for known risk factors, large geographical differences in rates of preterm delivery remain. Additional analyses to look at the effect of environmental and socioeconomic factors on gestational age found an increased gestational age in urban areas. Future research strategies could focus on investigating the urbanity effect to try to explain preterm delivery variation across countries with a very high human development index.

Keywords: epidemiology; premature; premature obstetric labor; preterm birth; preterm infant.

Figure 1

Cohort composition

Figure 2

Funnel plot of gestational age and population size plotted against the population mean gestational age [Color figure can be viewed at wileyonlinelibrary.com]

Figure 3

Preterm delivery rates across Sweden adjusted for known risk factors from a multiple linear regression model (both spontaneous and iatrogenic deliveries are included) [Color figure can be viewed at wileyonlinelibrary.com]

Figure 4

Preterm delivery rates significantly higher or lower than the population mean preterm delivery rate (binomial test P < 0.1, no multiple testing adjustment) [Color figure can be viewed at wileyonlinelibrary.com]

Figure 5

Weighted linear regression plots of environmental and socioeconomic municipality features and gestational age. Points represent municipalities, weighted by their number of deliveries (N) [Color figure can be viewed at wileyonlinelibrary.com]