Study of environmental, social, and paternal factors in preterm delivery using sibs and half sibs. A population-based study in Denmark.

OBJECTIVE The aim of this study was to evaluate the influence on preterm delivery of changes in putative genetic and environmental risk factors between two consecutive births. Low social status is a suspected risk indicator of preterm delivery, but the impact of social mobility has not been studied before. PARTICIPANTS The study uses national cohorts in which women act as their own controls. Subjects were identified by means of registries: 10,455 women who gave birth to a preterm child and had a subsequent live birth between 1980 and 1992 and 9849 women who gave birth to a child after 37 completed weeks of gestation and had a subsequent live born child in the same time period formed the cohorts. METHODS The risk of having a premature infant in the subsequent pregnancy was analysed in each cohort as a function of changes in male partner, residency, occupation, and social status between the two pregnancies. RESULTS There was a strong tendency to repeat a preterm delivery (18% v 6% in the general population). Social decline was associated with a moderate increase in the recurrence risk (OR: 1.22; 95% CI: 1.02, 1.47). In the reference cohort the risk of preterm delivery associated with changing from a rural to an urban municipality was 2.03 (95% CI: 1.14, 3.64). CONCLUSIONS Social decline and moving to an urban municipality may be associated with preterm delivery.