A source close to Dec and his wife Ali said: “They are both absolutely delighted to be expecting — especially as they have been trying for some time.”
The couple, who wed in 2015, have broken the happy news to Dec’s TV partner Ant McPartlin, who went into rehab this week after his drink-drive crash arrest.
It is thought 40-year-old Ali got pregnant shortly after Dec, 42, left the jungle in December following the last series of I’m A Celebrity.
But they kept it secret until they knew talent manager Ali had safely passed the crucial 12-week mark.
A source close to the pair, who wed in 2015 after knowing each other for a decade, said: “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 now is their baby and an experience that they see as the best of their lives.
“Dec and Ali are both absolutely delighted to be expecting, especially as they have been trying for some time.
“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 have told a few friends and Dec has told key people at ITV.
“Ali has also made some colleagues aware as she has been stepping back at work.
“She wants to make sure she takes it easy during the pregnancy.
“She knows it’s important for her to look after herself as much as possible.”
Dec’s best pal and TV partner Ant, who checked into rehab this week following his arrest on suspicion of drink-driving following a three-car smash, knows of the pregnancy.
Family-loving Dec, one of seven children born to Irish Catholic parents, has frequently spoken of his desire to have kids.
He and Ali, who began dating in August 2013, are understood to have started trying for a baby soon after they married.
Back then, Dec said having children was “the next stage of our lives”. He added: “I’m looking forward to that.”
Asked last year if he would like children, he said: “Absolutely at some point, when the time is right.
“Hopefully, as Ant says, if it comes along, then it would be great.”
On another occasion he said: “Kids are absolutely one of the things on the cards for us.”
The source said: “Dec comes from a big family himself and family has always been very important to him, as it is to Ali.
“So after getting married they both wanted to start trying as soon as possible.
“They were aware from watching others struggle to conceive — including Ant and his wife Lisa — that it might not be as easy as they hoped.
“And of course, the longer it took, the more they worried.
“So when Ali finally got a positive result it was the best news either of them could have hoped for.
“They are both beyond excited at the prospect of being parents.”
Dec and Ali met when she was employed by James Grant Management to look after his career with Ant, 42 — who ended up being best man at the pair’s wedding.
Sources said Ant will have mixed emotions over his friend’s happy news.
He and estranged wife Lisa, who announced the end of their 11-year marriage in January, had tried in vain for a child.