Mariah, 40, told Life & Style magazine: "Even before we announced it was twins, I was trying to keep everything gender-neutral because I didn't want to impose an identity on them too soon.
"There were fan contests on twitter about what gender they are and rumours about them being two boys or two girls - but nobody guessed this!"
Meanwhile, Nick - who broke the news the couple were expecting their first children back in December - confirmed they would be having mixed gender twins.
He told E! Online: "It's a touchy issue because you never want to say too much too early. If my wife says it's true, then it is."
The 30-year-old actor-and-comedian recently admitted he and Mariah have come up with some "unique" names for their unborn offspring.
Nick said: "They won't be getting crazy names like Carburetor or something. But they're definitely unique."