Showbiz
Rex Features
The TV chef and his wife previously spoke of their desire to have a large family, though the former model revealed she was experiencing difficulties conceiving as she suffers from polycystic ovaries.
The couple are already parents to Poppy Honey, six, and Daisy Boo, five.
A friend of the family told The Mirror: "They realised something was up when they were on holiday recently.
"They don't know if it is a boy or a girl yet. Jamie would love a boy as it's only girls in the house now.
"The girls are so excited, already thinking about being big sisters."
Last week, Oliver spoke of his desire to extend his family and said he and his wife may consider adoption.
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