Can pregnancy worsen arthritis?

Only the pain. Any extra load on an arthritic joint will exacerbate inflammation and pain; it will not necessarily over the term of pregnancy lead to further loss of joint cartilage (the source of "arthritis").
It can. It can but it usually doesnt. There are a lot of different kinds of arthritis some of which can worsen with pregnancy because of the affect of the hormones but sometimes the symptoms are temporarily relieved. With the 'garden variety" arthritis (osteo-arthritis) there is an increase in weight being put on the knees and back which along with the increase in ligament laxity can make symptoms worse.