He captured the image: “Happy Mother’s Day to all the mother’s (especially my mother), grandmother’s, dog mum’s and soon to be mother’s out there.”
The “soon to be” mothers comment had royal fans speculating he was referring to wife Alizée Thevenet.
However, the post didn’t include any images of James or Alizée but rather an image of the couple’s four dogs.
Royal fans immediately took to the comments, asking Mr Middleton to confirm the news.
If Alizée is pregnant, the child would be the third niece or nephew for the Duchess of Cambridge, who is aunty to her sister Pippa’s two children, Arthur and Grace.
James and Alizée were married in the Bormes-les-Mimosas in the south of France.
Around 50 guests attended the small event including the Duke and Duchess of Cambridge as well as Pippa and James Matthews.