The former Geordie Shore star is already mum to two-year-old Rox, who she welcomed with Casey back in 2019.
Speaking to new! magazine, the 29-year-old said: "We’re so excited and super happy. It was a bit of a shock but we’re just so thrilled!"
Marnie - who is 13 weeks along - also revealed she had been struggling with sickness second time around, adding: "With Rox I felt so healthy and glowing and I was a ray of sunshine, but this time I genuinely feel like the walking dead."
Casey added: "I’m really excited. Obviously you’re always going to be a little bit scared, too."
Marnie spoke about the moment she found out she was pregnant, explaining she was at work, filming for a TV show in September.
Marnie says she constantly felt sick and dizzy, and had bleeding gums - but put it all down to long filming hours.
"When I was away filming I was messaging Casey and I told him I felt really sick and my boobs were really hurting. I said it could be because I was running around and my muscles hadn’t been used in that way before, but Casey said he thought I was pregnant," said Marnie.
The couple also explained that while they do think having two will be "really hard" they plan on "cherishing" their kids' younger years, with Marnie explaining she's "99.9 per cent sure" this will be "their last pregnancy".
Casey and Marnie first met back in 2017 on MTV show Single AF.
Casey proposed to Marnie in 2020 and - after postponing their wedding due to the pandemic - the couple hope to tie the knot in the summer.
We're so happy for these guys! Congratulations to the whole family.
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