how do I walk a soon to be mother on leash?

I have a 3yr old Cavalier that I bought that is expecting! on 4/9

everytime I put a leash on her, she plops down and is determined not to move..

I have to carry her out and once on the grass, she will then gladly walk around and do her thing.. but until then, it is no go..

I don't want to force her because I don't want her to have extra stress but at the same time, if I always pick her up she will never learn..

what do I do..

BS