Specific HLA II alleles are associated with rheumatoid arthritis (RA) risk and others with protection. Risk-associated alleles encode similar amino acid sequences from 70 through 74 of HLA-DRβ1 (QKRAA, QRRAA, RRRAA), referred to as the "shared epitope" (SE) and protective alleles encode DERAA at the same location. Fetal-maternal cell exchange results in long-term microchimerism i.e. harboring small numbers of genetically disparate cells. Women with RA who lack the SE were recently found to harbor microchimerism with the SE more often than healthy women. This finding raises the question whether microchimerism with DERAA confers benefit against RA and is underscored by the observation that overall parity reduces RA risk. While there is currently no test for microchimerism with DERAA, we conducted studies to ask whether parity benefits women at risk for RA, either because they have the SE or lack the protective DERAA sequence. HLA genotyping was conducted for 310 RA and 571 healthy women. Parity was associated with reduced RA risk in women aged <45 years (RR 0.53, 95% CI 0.34-0.82) and further analyses examined this group. RA risk reduction with parity was greater among women with the SE than SE-negative women (RR 0.42, 95%CI 0.22-0.79 vs. RR 0.79, 0.38-1.64). Among women without DERAA, RA risk was significantly reduced with parity (RR 0.44, 95% CI 0.26-0.74) but not among DERAA-positive women (RR 0.95 95% CI 0.34-2.65). In summary, results indicate the effect of parity varied according to a woman's HLA-genotype, and women at increased risk of RA benefited most.