Objectives: Personal smoking and gender are determinants of adult rheumatoid arthritis. We assessed the independent and joint effects of maternal smoking in pregnancy and gender on the development of rheumatoid arthritis and other inflammatory polyarthropathies (RA&IP, ICD-9 code 714) in particular juvenile rheumatoid arthritis (JRA, ICD-9 code 714.3) in the first 7 years of life in a cohort of Finnish children born in 1987.
Methods: We identified 58 841 singleton births from the Finnish Medical Birth Registry and followed-up on them through other nationwide registries for 7 years. The birth registry provided categorical information on the mother's smoking during pregnancy: no smoking as a reference, low exposure (<10 cigarettes per day) and high exposure (>10 cigarettes per day).
Results: There were 75 cases of RA&IP yielding an incidence rate of 18.5 per 100 000 person-years. Of these, 31 were classified as JRA with an incidence rate of 7.6 per 100 000 person-years. In logistic regression, both the risks of RA&IP (adjusted odds ratio (OR) 2.10; 95% confidence interval (CI) 1.30-3.40) and JRA (3.03; 1.36-6.76) were increased in girls. High exposure to tobacco smoke increased the risks of RA&IP (2.57; 1.13-5.89) and JRA (2.98; 0.95-8.78) in girls, but not in boys. The adjusted ORs for girls with heavy exposure were 4.64 (1.94-11.07) for RA&IP and 6.76 (2.00-22.9) for JRA compared with unexposed boys.
Conclusion: This is an original finding of a potential effect of foetal exposure to tobacco smoke on the risks of RA&IP and JRA in girls.