The boy was reportedly born yesterday at 1.58pm, weighing 8lb and 9oz at the Lindo Wing in west London.
She looked absolutely stunning in a bottle green dress, that highlighted her baby bump, and a matching hat.
William and Kate, 36, are parents to Prince George, 5, Princess Charlotte, 3, and Prince Louis, 5 months. And, the mum to be previously revealed in her column that she is keeping up her fitness regime throughout and is particularly enjoying swimming.
"She swam parallel to the beach for about 100-130 feet, doing both the front crawl and backstroke".
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Like her sister, Pippa hasn't disclosed the sex of the baby. As a result, one of the biggest changes is that my lower body has tightened up. Reality is finally kicking in.
Middleton married financier Mr Matthews previous year.
"They'll be fantastic parents and we'll all be very close", he said.
Pippa Middleton attended Princess Eugenie's wedding just days before giving birth. "While the Lindo Wing was a functional hospital, it was a little bit like a hotel in terms of the service", the mother of two said.
A standard room at the Lindo Wing costs nearly $11,000 with deluxe rooms starting at $11,500.