Changes in Chlamydia trachomatis risk before and after union formation and separation among women of reproductive age

Background Not having an established relationship is associated with an elevated risk of Chlamydia trachomatis (CT) infection, but this might reflect selection into and out of unions. Although union formation and union separation are common events in reproductive age, little is known about changes in CT risk before and after these transitions.Methods We linked Finnish Population Register data to the National Register of Infectious Diseases and used fixed-effects linear probability models that account for all time-invariant confounders to examine changes in women's 6-month CT risk 3 years before and 3 years after entry into first cohabitation (n = 293 554), non-marital separation (n = 201 647) or marital separation (n = 92 232) during 2005-14.Results From 3 years to 1 year before first union formation, the 6-month risk of CT increased slightly, peaking at 1.27% immediately prior to union formation (95% confidence interval 1.22-1.31). It declined sharply following union formation, being only 0.40% (0.34-0.46) 6-12 months after union formation with little changes thereafter. Among women separating from non-marital unions, the risk increased from 0.50% (0.42-0.57) to 1.45% (1.40-1.49) around the time of separation and decreased following separation. The pattern of findings was relatively similar for marital separation, although the observed risks and changes were smaller in magnitude.Conclusions Our results based on longitudinal data and individual fixed-effects models indicate that the period immediately after separation may be causally associated with an elevated risk of CT. This suggests that recently separated women should be identified as a high-risk group for CT.