Ant McPartlin and Declan Donnelly started the show with a cabaret-style musical performance, which included the line: “And Alesha is expecting.”
The star was given a huge round of applause by the studio audience, telling them afterwards that she is “very happy” as David Walliams joked the baby is he, saying: “These things happen when we’re on tour.”
“It’s a lovely place to celebrate it,” Alesha said.
Alesha has been in a relationship with dancer Azuka Ononye since 2012 and they have one child, five-year-old Azura, together.
Monday’s show was the first of this week’s five live episodes, ahead of Sunday’s grand final.
The episode also marked Ant’s return to live hosting, after he took an extended break last March.
In his absence, Dec presented I’m A Celebrity... Get Me Out Of Here! alongside Holly Willoughby.