D.W.
I've linked you to the Prescribing Information approved by the FDA for Humira. Section 8.1 and section 8.2 should have the information you're seeking:
http://www.rxabbott.com/pdf/humira.pdf
According to the document, it's pregnancy category B which means it has been approved by the FDA for use in Pregnancy. However, there are serious side effects and a Black Box Warning (strongest warning from the FDA). I'd go through and read the entire document and discuss with a pharmacist as well as your specialists. I'd find out from your gastroenterologist what their concerns are vs. your rheumatologist and OB.
As for nursing, I'd not take the risk personally based upon the information in the document. I don't know if you can manage both your Crohn's and RA without this medication, but it doesn't appear to be worth the risk. When diagnosed with cancer after my daughter's birth, I lamented having to stop nursing her (I'd nursed her brother for the first year), but I realized having Mom was more important than having Mom's milk. In your case, if you can find safer alternative treatments or can manage without treatment, it may be a better option.
Good luck!