My DH has a condition they can't exactly put a finger on...and he's had it for 15 years or so. He was initially diagnosed with juvenile idiopathic arthritis, then ankylosing spondylitis, then Chron's, then when they realized it was none of those they diagnosed him with a HLBA27 Reactive arthritis. He's had surgery to remove inflamed tissue from one of his knees.
He did try methotrexate for some time, steroids, sulphasalazine, and many, many other things...nothing really worked. He is now on Remicade (admistered every 8-12 weeks by infusion), and that has FINALLY given him his life back and he has problems only when he waits too long to go in for his meds. Seriously, he has NO PROBLEMS...no swelling, no pain, his sed rate has been perfect...he calls Remicade his 'cure'. Remicade is not safe in pregnancy, but there are others in the same category of meds (Humira, Enbrel...maybe other new ones now) that are pregnancy category B...but I think there are a lot of unknowns since these drugs are so new. It certainly can't hurt to ask your rheumatologist about them though!
While Lexapro isn't as safe in pg, as Zoloft and a few others I *think*. I know it's hard because other meds might not work as well as what you are currently on. However, I do agree that benefits to you definitely outweigh the risks to baby as long as you can find a med you are comfortable with as some do carry higher risks in pregnancy.
A few websites I found, some say experts disagree whether rheumatoid arthritis reduces fertility in women :
http://www.webmd.com/rheumatoid-arthritis/ra-tips-for-living-8/pregnancy
http://www.rheumatoid.org.uk/article.php?article_id=83
http://arthritis.about.com/cs/preg/a/pregarth.htm
Good luck to you!!