ORs of developing RA overall and ACPA-positive/ACPA-negative RA according to breastfeeding. EIRA, Sweden, 2006–2014
| ACPA-status | Breastfeeding* | Ca/Co | OR (95% CI)† | OR (95% CI)‡ |
| RA overall | ≤6 months | 275/533 | 1.0 | 1.0 |
| 7–12 months | 275/574 | 0.93 (0.75 to 1.14) | 0.99 (0.80 to 1.23) | |
| ≥13 months | 334/842 | 0.77 (0.63 to 0.94) | 0.88 (0.71 to 1.08) | |
| Missing | 78/148 | – | – | |
| p-value trend | – | 0.0075 | 0.1919 | |
| ACPA-positive | ≤6 months | 194/533 | 1.0 | 1.0 |
| 7–12 months | 192/574 | 0.91 (0.72 to 1.15) | 0.99 (0.78 to 1.26) | |
| ≥13 months | 234/842 | 0.74 (0.59 to 0.93) | 0.86 (0.68 to 1.09) | |
| Missing | 52/148 | – | – | |
| p-value trend | – | 0.0086 | 0.2096 | |
| ACPA-negative | ≤6 months | 81/533 | 1.0 | 1.0 |
| 7–12 months | 83/574 | 0.97 (0.70 to 1.35) | 1.01 (0.72 to 1.42) | |
| ≥13 months | 100/842 | 0.83 (0.60 to 1.15) | 0.91 (0.65 to 1.27) | |
| Missing | 26/148 | – | – | |
| p-value trend | – | 0.2405 | 0.5446 |
↵*Breastfeeding duration categorised according to quartiles values among controls, merging the two highest categories.
↵†Adjusted for age and residential area.
↵‡Adjusted for age, residential area, smoking (pack-years) and alcohol consumption (low (including non-drinkers), medium and high).
ACPA, anticitrullinated protein antibodies; Ca/Co, number of cases/controls; EIRA, Epidemiological Investigation of Rheumatoid Arthritis; RA, rheumatoid arthritis.