Ayda revealed the birth of Beau Benedict Enthoven Williams to fans on Instagram.
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Robbie Williams and Ayda Field
David M. Benett via Getty Images
She wrote: “On this Valentine’s Day, we would like to celebrate love in the most awesome way.
“As with Coco, he is biologically ours, but born via our same incredible surrogate.
“We are so blessed to have our healthy son safely in our arms and are officially complete as a family.”
Alongside the message she shared a photograph of the baby’s feet next to those of his siblings.
The couple’s daughter Colette (Coco) Josephine Williams was also born using a surrogate in 2018.
They also have a daughter, Theodora, born in 2012, and son Charlton, born in 2010.
Following the birth of Coco, Ayda explained that she and Robbie endured a “long and difficult path” trying for another baby.
The 40-year-old said: “Family comes in all forms, and this little lady, who is biologically ours, was carried by an incredible surrogate mother, to whom we will be eternally grateful.”
Last year, Ayda admitted she was “desperate” to have a fourth child during an appearance on Loose Women.
Robbie and Ayda
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Robbie Williams arrives with his wife Ayda Field for the press night of Panto's On Strike, at the Opera House in Manchester.Picture date: Tuesday, November 16, 2010. Robbie has written a song for the show (Dave Thompson/PA Wire)
(02 of12)
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(03 of12)
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Musician Robbie Williams (C) girlfriend Ayda Field (R) and friend Jonathan Wilkes look on from the stand during the Barclays Premier League match between Arsenal and Manchester United at the Emirates Stadium on November 8, 2008 in London, England. (Photo by Shaun Botterill/Getty Images)
(04 of12)
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Robbie Williams with his blue suede shoes and blue checked shirt arrives in LA with girlfriend Ayda Field, Los Angeles, California - 16.09.09 (Michael Wright/WENN.com)
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Robbie Williams arrives with his wife Ayda Field for the press night of Panto's On Strike, at the Opera House in Manchester.Picture date: Tuesday, November 16, 2010. Robbie has written a song for the show (Dave Thompson/PA Wire)
(06 of12)
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Robbie Williams and Ayda Field at The BRIT Awards 2011 - departing from a boat at the Savoy Pier, London (WENN)
(07 of12)
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Robbie Williams and Ayda Field at The BRIT Awards 2011 - departing from a boat at the Savoy Pier, London (WENN)
(08 of12)
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Robbie Williams and Ayda Field at The BRIT Awards 2011 - departing from a boat at the Savoy Pier, London (WENN)
(09 of12)
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Robbie Williams arrives with his wife Ayda Field for the press night of Panto's On Strike, at the Opera House in Manchester.Picture date: Tuesday, November 16, 2010. Robbie has written a song for the show (Dave Thompson/PA Wire)
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Robbie Williams and Ayda Field at Spencer Hart clothing range launch party, Selfridges, London, Britain - 10 Feb 2011 (REX Features)
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The Royal Variety Performance, The London Palladium, London, Britain (REX Features)