The Black and Yellow hitmaker, real name Cameron Thomaz, proposed to model Rose earlier this year (12) and they confirmed rumours they are expecting their first child at the MTV Video Music Awards in September (12).
Khalifa now reveals the couple will make its romance official with a simple court house ceremony in a few days, with a bigger bash for family and friends next year (13), after Rose gives birth - so she can wear her dream wedding dress.
He tells New York radio DJ Angie Martinez, "We're gonna get, like, licensed marriage, in a couple of weeks we'll have that done.
"When you have the (big) ceremony that's when you can invite the family. She can fit into a dress and then... if we have a daughter one day, she might wanna give the dress to our daughter, but if she's pregnant then she can't do that."
And Khalifa admits there's another reason why his fiancee is keen to put off their big celebration until after she gives birth: "She wants to get drunk."
The couple began dating in 2010, after Rose split from Kanye West.