Estimated multivariable-adjusted hazard ratios (95% CI) of the association between perinatal factors and incident rheumatoid arthritis (RA) further classified by rheumatoid factor (RF) seropositivity.
| Nurses’ Health Study | Nurses’ Health Study II | Combined | ||||
|---|---|---|---|---|---|---|
| RF+ | RF− | RF+ | RF− | RF+ | RF− | |
| Preterm birth* | ||||||
| No | 1.0 (ref) | 1.0 (ref) | 1.0 (ref) | 1.0 (ref) | 1.0 (ref) | 1.0 (ref) |
| Yes | 1.0 (0.6, 1.6) | 1.1 (0.7, 2.0) | 0.8 (0.3, 1.7) | 1.5 (0.7, 3.2) | 0.9 (0.6, 1.5) | 1.2 (0.8, 1.9) |
| Breastfed† | ||||||
| No | 1.0 (ref) | 1.0 (ref) | 1.0 (ref) | 1.0 (ref) | 1.0 (ref) | 1.0 (ref) |
| Yes | 0.9 (0.7, 1.1) | 0.7 (0.5, 0.9) | 1.2 (0.8, 1.8) | 1.2 (0.7, 1.9) | 1.0 (0.8, 1.3) | 0.9 (0.5, 1.5) |
| Unknown | 0.9 (0.7, 1.2) | 0.9 (0.6, 1.2) | 0.5 (0.2, 1.6) | 1.6 (0.7, 3.6) | 0.8 (0.6, 1.2) | 1.1 (0.6, 1.8) |
| Duration breastfed† | ||||||
| None | 1.0 (ref) | 1.0 (ref) | 1.0 (ref) | 1.0 (ref) | 1.0 (ref) | 1.0 (ref) |
| ≤ 3 mo | 0.8 (0.5, 1.3) | 0.7 (0.4, 1.3) | 0.8 (0.4, 1.6) | 1.1 (0.5, 2.4) | 0.8 (0.5, 1.2) | 0.8 (0.5, 1.3) |
| 4–8 mo | 1.0 (0.7, 1.4) | 0.9 (0.6, 1.4) | 1.4 (0.8, 2.7) | 1.2 (0.5, 2.6) | 1.1 (0.8, 1.5) | 1.0 (0.7, 1.4) |
| ≥ 9 mo | 0.9 (0.6, 1.2) | 0.6 (0.4, 0.9) | 1.9 (0.9, 4.2) | 2.2 (0.9, 5.2) | 1.2 (0.6, 2.5) | 1.1 (0.3, 3.8) |
| Unknown | 0.9 (0.7, 1.2) | 0.7 (0.5, 1.0) | 1.3 (0.7, 2.4) | 0.9 (0.4, 2.1) | 1.0 (0.7, 1.3) | 0.7 (0.5, 1.0) |
↵* Multivariable preterm birth models adjusted for parents’ occupations (NHS only), race/ethnicity, mother smoked in childhood, father smoked in childhood, fetal exposure to cigarette smoke (NHSII only), and stratified by age and study time.
↵† Breastfeeding exposure models adjusted for parents’ occupations (NHS only), race/ethnicity, age mother smoked in childhood, father smoked in childhood, fetal exposure to cigarette smoke (NHSII only), birth weight, preterm status, and stratified by age and study time.