Thirty-five-year-old rapper announced the news during his December 20, 2012 concert at Atlantic City's Revel Resort Ovation Hall.
West paused in the middle of his set, yelling, "Stop the music for a second. Stop the music," before asking the 5,000 fans, "Can we make some noise for my baby mama right quick?"
The news quickly went viral.
The Keeping up With the Kardashians star and West have been dating since April 2012.
Kardashian allegedly didn't know West was going to make the proclamation. TMZ claims the couple had agreed to wait to make an announcement until she was showing.
This wasn't the first time West made an unilateral announcement about the couple: in August he tweeted that he had written his song, "Perfect Bitch," for Kardashian (who said she was "honored").
Legally, Kardashian is still married to basketball Kris Humphries--despite the couple filing divorce papers only 72 days after their extravagant wedding--because as their divorce has yet to be finalized.