Michael Phelps took his wife and his fetus out on the town on Tuesday -- treating the gang to a OneRepublic show! 

Earlier that day, Nicole Phelps announced she was preggo with baby #2 -- but it wasn't until she hit the concert that she decided to show off her baby bump. 

The Phelps' got to hang backstage with the band before the Phoenix show -- and tweeted about it, "These guys were amazing to see live the other night!! So talented!!!"

He's right. Counting Stars is a good song.