If he's just running up very enthusiastically to give you a hug, then he really doesn't even know that he's causing any problems. Explain that it is a problem, and show him a calmer/gentler way to approach you.
If you are playing rough together (wrestling, jumping on you), then he's getting into the spirit of things just a little too much. Explain that playtime can still happen, but it needs to be calmer/gentler (and he needs to listen when somebody says "stop, that's too rough").
If you want to be open about the upcoming sibling, explain that he needs to stop because you're pregnant and you need him to be more gentle.
It hurts me when you run into me like that, and you can also hurt the new baby that's growing. Please play more gently now that I'm pregnant.
If you're not telling him yet for whatever reason, just leave pregnancy out of the discussion. That last impact did cause you pain even though the fetus was safe.
It hurts me when you run into me so hard. Please play more gently.
If he continues injuring you, put him in time-out every single time and explain exactly why ("You can't play so rough. Take a break to calm down.") -- it shouldn't be long, just a minute. That's enough to convey that hurting his mom causes playtime to end.
More importantly, always praise him when he does play gently and isn't rough. ("Thanks for that careful hug!")