Pete's rep confirmed their happy baby news to Reveal this week, saying the parents-to-be are "over the moon".
The timing of Emily's pregnancy coincides with Katie's fourth pregnancy: she is expecting her first child with third husband Kieran Hayler.
Katie, who has two children with Peter and one son from a former relationship, has been keeping a noticeably low profile during her pregnancy, with no pictures of her bump appearing in the media until recently.
In her weekly column for The Sun, model Katie said that she is "sure" Peter and Emily will follow in her footsteps in keeping their pregnancy private.
"I'm delighted that my ex-husband Pete's lovely partner Emily is pregnant too," she wrote. "I'm sure they'll take the same private approach as me, as I know she is sensibly not interested in the media business."
"Good on her," she added. "And good luck to them both and their little bump."
Katie first sent her congratulations to the couple earlier this week, saying: "Genuine congratulations to Pete and Emily and I hope they are very happy."
As for her own decision to keep her pregnancy private, Katie wrote: "I decided three years ago to withdraw my children from filming and photoshoots as I want them to grow up like any other child away from the spotlight."
Katie is mum to son Harvey, 11, from her relationship with footballer Dwight Yorke. She shares son Junior, 8, and daughter Princess, 6, with Peter.
Peter, meanwhile, thanked fans for their congratulatory messages earlier this week with a simple tweet saying: "Thanks to all of you for your awesome messages. You are great :))))"
Speculation that Emily was pregnant arose earlier this year when Peter was seen holding a protective hand over her stomach while at a red carpet event in London.
'Mysterious Girl' singer Peter recently admitted in an online video that he wanted to have another baby, adding that he ideally wanted to be married or engaged first.
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