The yummy mummy announced the happy news on Monday night, tweeting: "I'm sooo happy to tell you all - we've just had a baby boy! Chester William Baldwin made a grand entrance at 8lbs 12. He's just perfect."
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Holly Willoughby and husband Dan Baldwin are now the proud parents of three
This Morning presenter Holly, 33, and Dan have been married since 2007 and are already parents to five-year-old Harry and Belle, three.
Hours after welcoming her baby boy, Holly made a quick call to This Morning to speak to her co-host Philip Schofield and Amanda Holden – her maternity cover – about the birth.
"It was very different to my other two," she said. "I had a water birth, that was amazing.
"He was a really big boy, 8lbs 12oz. He is so delicious, I can't stop looking at him. He is perfect."
Of her newborn son's name, Holly revealed, "It was a name we had for ages. When we had Harry we were umming and ahhing. Although when the kids came in to meet their little brother before going off to school, and I said his name is Chester, Harry said, 'No, I am going to call him Frank!'
"We are all well, and all just doing that lovely baby bubble," she added. "It's just wonderful."
This Morning presenter Holly Willoughby announced her pregnancy in March
Holly was thrilled as she revealed the news of the pregnancy in March, saying it was unexpected.
"Surprise Surprise," she wrote on her website, making a jokey pun on the title of her other popular TV show. "(And it was a surprise!!) I'm really happy and excited to tell you that Dan and I are expecting baby number 3!!"
"We feel blessed and very lucky to be giving Belle and Harry a little brother or sister later this year. Xxx"
The new arrival means that Holly, who admitted to be "perennially broody" last year, is well on her way to her goal of having a big family.
If she finds it hard juggling motherhood with her busy job - she is also team captain on ITV comedy panel game Celebrity Juice - it doesn't show.
In fact, the bubbly blonde says her chosen career actually gives her more family time.
"People think I work a lot more than I do," she recently told The Observer. "I think because you're in people's living rooms every day they're like: 'Oh my God, you're always on the telly,' but it's like, 'Yeah but you always have to go to work every day nine till five whereas I finish at 12.30pm and then I'm home.'"