PARITY AND RISK OF RHEUMATOID-ARTHRITIS IN FINNISH WOMEN

Several recent case-control studies have suggested that nulliparity is a risk factor for rheumatoid arthritis (RA). We studied parity for it s association with the incidence of RA in a cohort of adult Finnish wo men examined by the Mobile Clinic of the Social Insurance Institution in 1966-72. The 15 441 women were greater than or equal to 30 yr of ag e and had neither arthritis nor a history of it at the time of the bas eline examination. They were followed up by examining records kept at the population register of the institution, to identify patients entit led to free anti-rheumatic medication. Up to late 1989, 269 women had developed RA. Of the incident cases, 176 were rheumatoid factor (RF)-p ositive. Compared with nulliparous women, the age-adjusted relative ri sks (95% confidence intervals) of parous women developing RF-positive RA, RF-negative RA or either type were 1.26 (0.82-1.92), 0.80 (0.48-1. 32) and 1.06 (0.77-1.46), respectively. The number of childbirths pres ented an n-shaped relationship with the risk of RF-positive RA, but th ere were distinct differences between subpopulations. In female farmer s the association was inverse, but in housewives and in other occupati onal groups (mainly employees) it was clearly positive. The results ch allenge the view that there is a causal relation between parity status and the development of RA.