Well, I am qualified to comment and this is not a topic for which you should be seeking an answer from strangers. Her gyn can establish the likelihood that the past miscariage was related to anything other than a "defective" foetus - the most common cause. Only one act in four during a woman's fertile time results in a live birth. Many of the others are unknown miscarriages.
She, and you, should discuss this with the gyn. Based upon past patients, you will likely have some deep-seated concern no matter what you are told. That is normal. Perhaps you two should brush up on your oral skills and, if allowed by the gyn, learn a position that would keep pressure off her tummy.
I recommend that she lie on her back with her legs draped over you lying cross-wise on your side. That gives each of you complete access to all critical parts of either of you. If the gyn clears you for sex, this is the least risky position.
Brandye