Objective. Previous studies have evaluated the correlation between rheumatoid arthritis (RA) risk and pregnancy history, with conflicting results. Fetal cells acquired during pregnancy provide a potential explanation for modulation of RA risk by pregnancy. The present study was undertaken to examine the effect of parity on RA risk.Methods. We examined parity and RA risk using results from a population-based prospective study in Seattle, Washington and the surrounding area and compared women who were recently diagnosed as having RA (n ‫؍‬ 310) with controls (n ‫؍‬ 1,418). We also evaluated the distribution of parity in cases according to HLA genotype.Results. We found a significant reduction of RA risk associated with parity (relative risk [RR] 0.61 [95% confidence interval 0.43-0.86], P ‫؍‬ 0.005). RA risk reduction in parous women was strongest among those who were younger. Most striking was that RA risk reduction correlated with the time that had elapsed since the last time a woman had given birth. RA risk was lowest among women whose last birth occurred 1-5 years previously (RR 0.29), with risk reduction lessening progressively as the time since the last birth increased (for those 5-15 years since last birth, RR 0.51; for those >15 years, RR 0.76), compared with nulliparous women (P for trend ‫؍‬ 0.007). No correlation was observed between RA risk and either age at the time a woman first gave birth or a woman's total number of births. Among cases with the highest genetic risk of RA (i.e., those with 2 copies of RA-associated HLA alleles), a significant underrepresentation of parous women versus nulliparous women was observed (P ‫؍‬ 0.02).Conclusion. In the present study, there was a significantly lower risk of RA in parous women that was strongly correlated with the time elapsed since a woman had last given birth. While the explanation for our findings is not known, HLA-disparate fetal microchimerism can persist many years after a birth and could confer temporary protection against RA.

Objective. To examine fecundity prior to disease onset in women with rheumatoid arthritis (RA).Methods. Two hundred fifty-nine RA patients with an opportunity for pregnancy were compared with 1,258 control women as part of a prospective cas+ control study of recent-onset RA in women.Results. A 12-month or longer interval to pregnancy was found in 42% of RA cases compared with 34% of controls (odds ratio = 1.44, 95% confidence interval 1.10, 1.91). This difference was not accounted for by an older age when attempting to become pregnant among the RA cases compared with the controls. The

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As part of a prospective case‐control study of newly diagnosed rheumatoid arthritis (RA) in women, we identified all cases of probable, definite, or classic RA diagnosed in 1987–1989 in 18–64‐year‐old women who were members of a health maintenance organization based in the Seattle, Washington area. Using both the 1958 and the 1987 American Rheumatism Association criteria for the diagnosis of RA and enrollment data from the health maintenance organization, we calculated the incidence by age and diagnostic class. Rates of RA incidence in women increased steadily with age. The incidence of probable, definite, or classic RA ranged from 13.1 per 100,000 person‐years at risk for 18–29‐year‐old women to 82.1 per 100,000 person‐years for 60–64‐year‐old women. The overall incidence rate, age‐adjusted to the 1980 US female population, was 27.9/100,000 person‐years. The overall incidence rate for definite/classic RA, age‐adjusted to the 1980 US female population, was 23.9 per 100,000 person‐years. When compared with adjusted rates of incidence of definite RA in Rochester, Minnesota, in 1950–1974, the incidence rates we found were 44.7% lower. Methodologic differences, changes in diagnostic criteria, and a declining incidence of RA among women over time may all be partial explanations for these results. The possible effects of reproductive factors, including oral contraceptives use, are discussed.