Machine Gun Kelly has sparked rumours his fiancee Megan Fox could be pregnant by dedicating a song to their "unborn child" at the Billboard Music Awards

At the start of the performance, he strummed his guitar and said: "I wrote this song for my wife," before adding: "And this is for our unborn child."

An onlooker told E! News: "People in the audience turned to each other perplexed and said, 'Wife?'"

The pair are engaged to be married but have yet to confirm if they have tied the knot in secret.

However, Machine Gun Kelly has previously referred to Megan as his spouse, despite having yet to walk down the aisle together.

Speaking on the red carpet prior to the show, the singer-and-rapper revealed he was planning to celebrate Megan's birthday afterwards.

He said: "It's my fiancée's birthday at midnight so we are celebrating her wonderful night."

The 'Transformers' actress then added: "I'm boring I don't drink, but I'm sure he'll go off for me."

Megan is mum to three boys, Journey, five, Bodhi, eight, and Noah, nine, with her ex Brian Austin Green, while Machine Gun Kelly has a 12-year-old daughter named Casie from a previous relationship with Emma Cannon.

The couple have yet to comment on the baby rumours, but a source claimed that although the pair would love to start a family together, they plan to wait a few more years.

The insider said: "Megan would love to have a baby with MGK ... they've already discussed it ...

"(But) they both want to wait a couple of years. They're having way too much fun right now.'"