Table 2 Reproductive factors and the risk of rheumatoid arthritis for all cases and rheumatoid factor (RF)-positive and RF-negative cases in univariate analyses
Cases (n)Person-yearsat riskOR (95% CI)
All (n = 136)RF +ve (n = 91)RF −ve (n = 42)
No breast feeding43926ReferentReferentReferent
Breast feeding 1–12 months (vs not)7420650.74 (0.45 to 1.20)1.07 (0.58 to 1.95)0.40 (0.16 to 0.97)
Breast feeding ⩾13 months (vs not)197040.46 (0.24 to 0.91)0.61 (0.28 to 1.36)0.30 (0.08 to 1.17)
No OC use631711ReferentReferentReferent
Ever used OCs6717951.03 (0.63 to 1.67)1.07 (0.58 to 1.97)0.98 (0.42 to 2.27)
Each child bornNANA0.87 (0.71 to 1.06)0.87 (0.68 to 1.11)0.77 (0.54 to 1.11)
  • NA, not applicable; OC, oral contraceptive;+ve, positive; -ve, negative.