Birth order, sibship size, and housing density in relation to tooth loss and periodontal disease: a cohort study among Swedish twins.

For diseases with an infectious etiology, birth order may dictate the age of exposure to childhood infection, while sibship size may be a proxy for the probability of exposure. The authors examined whether birth order, sibship size, and childhood housing density affect risk of tooth loss and periodontal disease. The study included 28,690 adults aged > or = 42 years who were participating in a 1998-2002 follow-up of persons listed in the Swedish Twin Registry. Logistic regression was used to calculate odds ratios and 95% confidence intervals, with adjustment for age, sex, education, and smoking and mutual adjustment for family composition (sibship size and/or birth order). Tooth loss and periodontal disease affected 8% and 19% of the twins, respectively. Each additional sibling increased the odds of tooth loss by 10% (95% confidence interval (CI): 1.06, 1.15) and the odds of periodontal disease by 5% (95% CI: 1.02, 1.08). Later birth order was associated with lower odds of periodontal disease. Each additional person per room in the childhood home increased the odds of tooth loss (odds ratio = 1.28, 95% CI: 1.03, 1.60) but lowered the odds of periodontal disease (odds ratio = 0.65, 95% CI: 0.48, 0.89). These findings are compatible with the hypotheses that adult oral diseases are associated with the probability of exposure in childhood and that earlier age at exposure lowers risk.