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Unborn babies can sow the seeds for rheumatoid arthritis in their mothers - and the dads might be to blame. Rheumatoid arthritis is an autoimmune disease, meaning the body's immune system turns on itself. In this case, it causes painful, swollen joints. Women are three times as likely to develop the condition as men, and seem to be especially vulnerable soon after pregnancy. A mother exchanges cells with the fetus while it is in the womb. "For most women, shortly after you give birth, the fetal cells clear up," says Giovanna Cruz, an epidemiologist at the University of California at Berkeley. "But in a subset of women they actually persist for decades." In these women, the fetal cells are effectively incorporated into their bodies, a process known as microchimerism.