Honestly your tone will make a massive difference, go and get a congratulations card or ballon or something, and go in showing scan pictures and your excitment or take them out for a meal, say we have something to celebrate. Make it positive and show your happy or they will react to your mood.
I'm 32, married with 4 kids, when I told my mum about my 4th I was in shock (it was a surprise) and I felt like I had been a big unresponsable because here was I about to have my 4th in as many years haha (I had twins in the middle), and she totally reacted to my mood, and was a bit upset, this is my mum who lives for her grandkids, everything is about them and she loves them to bits! If I had gone in, excited she would have been thrilled, I regret letting my anxiousness ruin that for us.
Go in happy, be prepared they may ask questions about your future plans, but seem up beat. Depending how much you want them involved invite them to your next scan etc.
Be warned though, when I told my nana she said "oh bloody hell, what did you go and do that for again" haha!
The flip side is, no matter how bad their intial reaction is I imagine their will be over joyed when baby is here, even my nana whose actions have been anything but supportive is over joyed by her great grand kids.
Take a deep breath and put a smile in your face. It's GOOD news! Congrats!