Fans began asking the couple about the speculation after the cover of Life & Style magazine showed a picture of them with the caption: "We're having a baby."
However, Shelton quipped on his Twitter page that he was "confused" about the headline.
"Look, I know I've gained some weight but the test was negative!!! #futfack," he joked.
Shelton also had a reply for a follower who asked if Lambert was expecting, saying: "I hope not.. She's passed out with a bottle of Bacardi in her hands!!!"
Meanwhile, Lambert took to her own Twitter page to refute the rumours, saying: "Wow! I just read the headline! "we are having a baby" yay! @blakeshelton we're getting a new puppy?:)"
She later added: "Don't believe everything you read! I'm having a big 'randarita' right now In honour of the baby we are not yet having!"
Lambert recently said that she does not plan to have children "for a while", joking that she thinks of her dogs as her kids.