While Peter also shared a happy snap of the pair, writing: "Soon to be a family of three and we are rapt! Good thing I’m already match fit with the whole no sleep thing."
A number of celebrities took to the 'gram to send their congratulations, including PE Nation boss and mum to one boy, Pip Edwards, who wrote: "Excellent news mumma."
Michael Pell, producer of Today's main competition at Seven's Sunrise, wrote: "Congratulations you two! Such good news!"
While another competitor over at Studio 10, Sarah Harris, who is also a mum of two boys, added: "BOYS ARE THE BEST!!!!!!! Yay you two. You’ll be brilliant parents."
Sylvia's fellow Nine star Leila McKinnon, who is married to the former CEO of Nine David Gyngell, wrote: "The best news! You two are going to be such amazing parents. We are thrilled for you both."
Sylvia's pregnancy has been speculated in recent weeks, in particular when she wore a belt to hide her tummy at the Logies and when she chose to jog Sydney's City2Surf marathon in late July. She said at the time that it was because of a hip injury.
"I will be doing my slowest time at the City2Surf [14km] this year," she said.
"What’s your thoughts on a child? Is everyone trying to push you now that you’re married?” radio shock jock Kyle Sandilands asked the journalist.
"Oh that’s been happening from probably the first day of our honeymoon I think?" Sylvia coyly replied. "Oh well, you can’t really plan these things, can you? It happens when it happens."
Peter, 37, younger brother of axed Today star Karl Stefanovic, met Sylvia at work when they were put together for co-hosting duties on The Weekend Today Show.
He popped the question on holiday in France in 2016, getting down on one knee at a vineyard in Bordeaux.
In April 2017, they said their "I dos" in an intimate ceremony at Kangaroo Valley in New South Wales.
The pair weathered a difficult period towards the end of 2018 when Peter was axed from Nine following the Uber-gate scandal when he was overheard by an Uber driver whinging about the network and his colleagues with his brother Karl.
Sylvia and Karl were also let go from the Today breakfast show.
However, things have been looking up for the pair again after Peter landed a gig at Sky News' flagship program, First Edition.
While, Sylvia joined Channel Nine's prime time news program, A Current Affair.
"It has been a really interesting and fun year," she said at the Logies earlier this year, admitting that she's loving her new TV gig. "I love the team, a great team."