The first two books, Bridget Jones's Diary and The Edge of Reason, have sold 15 million copies worldwide and spawned two movies, both staring Renee Zellweger in the titular role.
Fielding also revealed that the digital age will have opened up a whole new avenue for bumbling Bridget to air her social anxieties.
"I sort of lost my voice with Bridget for a long time after the unexpected success when it first came out," said Fielding, who now lives in London after residing in Los Angeles for the past decade.
"When I first wrote it I didn't think anyone would read it. It was just a column in The Independent, so it was very easy to write and be very honest - and then I got all self-conscious because I knew so many people would read it."
She also explained why she decided to bring Bridget back.
"I just found last Spring I had new stuff I wanted to say and things that were making me laugh," the author said, "Things that didn't exist when I last wrote, like emails and texting. The way life is lived through texting and Twitter, and a whole new idea for a phase of her life that I can't jinx by describing it."
'SHE'S NEVER REALLY GOING TO CHANGE'
And we can expect a shift from Bridget's trademark diarising of her booze and calorie intake.
"It's more like 'number of Twitter followers: 0. Still no followers. Still no followers'. But she has grown up. My life has moved on and hers will move on too," said Fielding.
"She's still trying to give up [drinking and smoking], she's still on a diet. She's trying a bit harder, and is a bit more successful, but she's never really going to change."
Fielding, who is also working on a third Bridget Jones movie (which is not based on a novel) and a Bridget Jones musical, added that the new book would explore "this whole world of internet dating where there's all these cyber-presences who aren't actually real.
"And texting - people can have entire relationships via text which are in some ways emotionally fulfilling and serve the purpose of a boyfriend, apart from sex," she said.
"In the same way that the first Bridget book looked at the way a thirtysomething single woman was branded as a tragic spinster and we got the idea of singleton, it's looking at later phases in life where you get branded as a certain thing," she added.
"It's a bit scary bringing out another one but I'm really enjoying writing it."
Fielding would not reveal what has happened to Bridget's love interests, socially awkward barrister Mark Darcy and dastardly ladies' man Daniel Cleaver.
"Some characters remain and some may have disappeared," she said. "They'll still be presences in the book. Like all of us you keep your friends, people stay in your life, but everyone's life moves on."
'THE STAKES ARE VERY HIGH'
The long-gestating third film - tentatively titled Bridget Jones's Baby - is due to start shooting next year.
Fielding told the BBC it is based on "a different, earlier story, not based on a novel - it started with some columns I wrote in the Independent about six years ago.
"I think with a third instalment [of the film] the stakes are very high. We all go back a long way, we're all really fond of each other and everyone really wants to get it right, but that means there are a lot of voices, a lot of writers.
"Getting Bridget right, I always think it's like a little duck - in the end it needs to skim effortlessly along the surface, but underneath the legs are going really, really fast."
The Full Monty's Peter Cattaneo will direct, while Fielding is co-writing the screenplay with David Nicholls.
Listen to Fielding's Woman's Hour interview here.
FAST GOSSIP
And finally...
* Elton John and David Furnish preparing for baby number two?
* Roger Moore on why Daniel Craig is the best James Bond.
* Halle Berry denied request to move daughter to France.
* Lindsay Lohan bails on scheduled interview with Barbara Walters.
* Vanessa Paradis says she will never reveal why she and Johnny Depp split.
* Mick Jagger's love letters to go under the hammer.
* Matthew McConaughey drops major pounds for new movie.
* Mark Wahlberg will talk to robots in Transformers 4.
* The way they were: Justin Bieber and Selena Gomez before shock break-up.
* Steven Spielberg rules himself out of directing new Star Wars movie; Little Miss Sunshine Screenwriter Michael Arndt to pen EPISODE VII.
* Quote of the day: "I read a comment on YouTube that I thought would upset me 'Test pilot for pies' - but I've always been a size 14-16 and been fine with it. I would only lose weight if it affected my health or sex life, which it doesn't." - Adele hits back at weight critics in new (unauthorised) biography.
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