It is thought the pair are delighted following the joyous news.
A source close to Ali, 40, said: “They are both absolutely delighted to be expecting – especially as they have been trying for some time.”
The Ant and Dec insider added: “This has come at a difficult time for them both, given everything that Ant is now going through and the pressure that has put on Dec in the past week.
“But their priority is now their baby and experience they see as the best of their lives.
“They have both always wanted a family but worried that it might not happen for them.
“They have been keeping the pregnancy a closely-guarded secret, but now that Ali is safely over 12 weeks, they are starting to let people know,” they added to The Sun.
What’s more, it is thought that Ali will step back from some work commitments in a bid to “take it easy”.
Express.co.uk has contacted a representative of Declan Donnelly, who declined to comment.
In 2013, Dec commented that having children would be “the next stage of his life” with Ali.
The beloved presenter was quizzed on whether he could see little Donnellys in his future last year, to which he insisted: “Absolutely at some point, when the time is right.
“Hopefully, as Ant says, if it comes along, then it would be great.”
Dec met Ali when she was employed by James Grant Management to look after his and Ant’s career.
The pair tied the knot in 2014 in a star-studded bash in Newcastle.
Holly Willoughby, Phillip Schofield and several more ITV stars were in attendance.
Of course, life-long pal Ant was his best man for the romantic affair.
Dec and Ali posed for cameras and fans outside the church where they shared a cute smooch.