Would I be the asshole if I called her out and asked her "so, how many years pregnant are you?"

For HR reasons I strongly don't recommend this approach. Act excited about her being pregnant because you saw her park in the expecting car, and say you don't want to make her uncomfortable but you're curious about her due date and if she's hoping for a boy or girl. Get all your coworkers to approach her together, everyone really excited. Someone bake a cake that says congratulations. Shame the hell out of her.

I majored in passive aggressive confrontations in the workplace. HR is never going to write you up if they think you're a nice person coming from a good place.