I would like to add a quick anecdotal note to the discussion.  I have had
arthritis (rheumatoid although I have always tested negative) since I was
16.  During my 3 pregnancies I was in remission until the third trimester
when it would flare up badly as the pregnancy progressed.  Lactation was a
period of blissful remission, with the flare ups returning after weaning.
In my case that meant I spent most of my 20s and 30s in remission, as I
spent most of that time pregnant, nursing or both.  It was extremely
disheartening to have the pains return. So I am of the belief that hormones
can contribute significantly to the path of the disease.  I would also
caution that arthritis comes in a tremendous number of variations, most of
which are not well researched or recognized.  This is not an issue that can
be generalized but each mother must be monitored individually. :)