My 2 years old shih tzu is 4 weeks pregnant. Ever since, she really loves climbing stairs and seems to be her territory. But I'm a bit worried if it might affect her pregnancy. I want to assure the best for her so I would restrict her to climb the stairs if needed.
I agree with trond's comment: Let the dog decide.
Pretty much all her behavior during the pregnancy will be instinct based. If there are things she shouldn't do, she'll "know" that. If there's something that hurts or is uncomfortable, she'll most likely avoid it or at least show clear signs of discomfort.
This doesn't mean you can't help the dog is make things more comfortable, but overall I'm pretty sure she'll handle just fine. Just make sure she finds herself a calm corner for herself and the puppies later on that's actually puppy safe.