Years after Hench’s study 70 years ago, which found a protective effect of pregnancy in patients with rheumatoid arthritis (RA) and an improvement in RA during pregnancy, several studies have suggested that pregnancy status may be associated with the risk of RA, but this is still controversial. A recent prospective population-based study supported by the National Institute of Child Health and Human Research and funded by the National Institutes of Health showed that the risk of RA was significantly reduced in menstruating mothers compared to non-mothers, with the lowest risk of RA 1-5 years after delivery. The study included women aged 18-64 years in Washington, D.C., and surrounding areas in the United States, with 310 patients with RA and 1418 controls. The relationship between the number of births and the risk of RA was studied. The results showed that the risk of RA in menstruating women decreased by 39% (P = 0.005) and that the younger the woman was, the lower the risk of RA. The duration of time after the last delivery was associated with the risk of RA, and the risk of RA was lowest in menstruating women 1-5 years after the last delivery, with a reduction of 71% (RR 0.29) compared with women who had not given birth; thereafter, the risk of RA decreased gradually, with a reduction of 49% (RR 0.51) 5-15 years after the last delivery and a reduction of 24% (RR 0.76) >15 years. However, the risk of RA was not related to age at delivery and number of deliveries. CONCLUSIONS: The risk of RA is significantly reduced in menstruating mothers and this risk is associated with the length of time interval since the most recent delivery, and HLA haploid fetal microchimerism can persist for several years after the patient’s delivery may be a possible mechanism for the patient’s reduced risk of developing RA in the short term.