The X Factor winner posted a picture of her first ultrasound scan on Twitter on Thursday morning alongside the caption: "Meet peanut!"
The unexpected news means that Sam, who is ten weeks pregnant, has had to reschedule her solo tour.
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"I was really upset because I thought I'd let everyone at my management and the record label down, " the former prison officer told The Sun newspaper.
"But they were really good about it. I was going to go on tour solo at the end of October - but I have managed to move it until January."
The new dates will give the mum of two – Brooke, eight, and 5-year-old son Tommy with husband Craig, 40 – time to prepare for the arrival of a new addition to the family later this year.
Sam revealed that upon learning of the pregnancy, she turned to her X Factor mentor Sharon Osbourne.
"I'm close to Sharon," she said. "We speak quite a lot. She’s my confidante and has known about the pregnancy. She’s been great."
Sam, 36, was crowned the latest winner of ITV talent show X Factor in December, beating fellow finalist Nicholas McDonald after receiving more than one million public votes.
"I don't know what to say," said the overwhelmed star at the time. "I just want to say that I love that boy to pieces, Nicky McDonald."
Adding that she was "just so grateful," Sam said, "Thank you so much everybody. I really am appreciative."