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The Welsh actress, 44, announced the news on Twitter, writing: "Well this is a surprise! Baby number 4 on the way!
"It's going to be a busy Christmas!"
Joanna and husband John Thornton are already parents to three children - an eight-year-old daughter and two sons aged four and six.
And friends and followers have been quick to congratulate the Gavin and Stacey star, with one person commenting: "I've got four kids, hugest congratulations… get some kip in now, you’ll need it!! Biggest loves to you and your family."
While another said: "Congratulations, I have four children and the house is definitely always busy and noisy but wouldn’t change it for the world."
And a third added: "My third babba was a complete surprise and she was born on Boxing Day. Best Christmas present ever, congratulations."
And actress Sally Lindsay wrote: "Woo hoo!! Wow Jo! Many congratulations."
In other Gavin and Stacey news, last month, Larry Lamb teased the possibility of more episodes.
The actor has been quizzed on whether the show will be back on many occasions since the last special aired in 2019.
And he dropped the biggest hint yet when asked by Metro recently, simply replying: "I think it would be pretty short-sighted if there wasn't.
"I'd be happy to do more. Everybody that's involved with it sees it as a team event. If they're doing more, you're part of it."
And Larry isn't the only cast member to suggest a new episode was inevitable at some point.
Speaking during his podcast, 'Brydon &', Rob Brydon told guest Sheridan Smith: "I think they might do one more special because they left it hanging, didn't they, with the proposal?
"So it would seem strange if they didn't. But they should crack on if they are going to do it."
Come on guys, give the people what they want!
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