The relation of body mass index, body height, and parity to pyelonephritis: A nationwide population-based cohort study of over one million parous women (1997-2018)

Objectives: We aimed to explore the association between various physiological factors and pyelonephritis in parous women. Methods: Swedish nationwide registers were used to identify 1,073,467 parous women aged 15-50 years. The study period began in 1997 and ended in 2018, or at the first incidence of pyelonephritis, death, or emigration. Cox proportional hazards methods were used to determine hazard ratios (HRs) and 95% confidence intervals (CIs) of pyelonephritis in relation to body mass index (BMI) and body height adjusted for age, parity, and individual-level sociodemographic variables. Results: A total of 21,625 women (2.0%) were diagnosed with pyelonephritis during follow-up. In the fully adjusted model, low BMI (<18.5) was associated with pyelonephritis (HR 1.13, 95% CI 1.06-1.21). Parity and certain sociodemographic characteristics were also independently associated with pyelonephritis. Body height <161 cm was associated with pyelonephritis (HR 1.12, 95% CI 1.09-1.16) in all but the fully adjusted model. Conclusion: Low BMI and body height seem to be associated with uncomplicated pyelonephritis. More research is needed to verify these findings and identify possible mechanisms.