| Birthweight | RA cases | Person-years | Age-adjusted RR (95% CI) | Multivariable RR (95% CI) adjusted for perinatal confounders* | Multivariable RR (95% CI) adjusted for perinatal confounders and RA risk factors† |
| <2.5 kg | 31 | 174 782 | 0.9 (0.6 to 1.4) | 0.9 (0.6 to 1.4) | 0.9 (0.6 to 1.5) |
| 2.5–3.175 kg | 91 | 495 776 | 1.0 (0.7 to 1.2) | 0.9 (0.7 to 1.2) | 1.0 (0.7 to 1.3) |
| 3.2–3.85 kg | 139 | 716 737 | 1.0 (ref) | 1.0 (ref) | 1.0 (ref) |
| 3.9–4.54 kg | 28 | 174 632 | 0.8 (0.5 to 1.2) | 0.8 (0.5 to 1.2) | 0.7 (0.4 to 1.1) |
| 16 | 36 249 | 2.2 (1.3 to 3.7) | 2.1 (1.2 to 3.5) | 2.1 (1.2 to 3.6) | |
| Not sure/data missing | 73 | 400 594 | 0.9 (0.7 to 1.2) | 0.9 (0.6 to 1.3) | 0.8 (0.6 to 1.1) |
Significant entries in bold.
*Adjusted for potential confounders that occurred during the perinatal period: age, maternal history of diabetes, father’s occupation at age 16 (proxy for socioeconomic status at time of birth), prematurity, history of parents’ smoking (proxy for fetal second-hand smoke exposure), mother’s body shape at age 50, region of country lived in at birth.
†Adjusted for potential perinatal confounders above plus risk factors for RA, including pack years smoked, age at menarche, use of oral contraceptives, use of post-menopausal hormones, total lifetime breastfeeding, BMI at age 18 and a history of diabetes mellitus.
BMI, body mass index; RA, rheumatoid arthritis; ref, reference group; RR, relative risk.