Alex Apati of Ladbrokes said: "Baby mania is in full swing in the Royal Family right now so it's no surprise to see odds on a fourth Will and Kate baby so short."
The Cambridge parents already share three children - Prince George, 5, Princess Charlotte, 3, and Prince Louis, 6-months.
Earlier in the year, Kate’s former school friend Jessica Hay exclusively told New Idea how she makes motherhood look easy.
"They’re a family of five now, so I would expect them to have one more baby to make a nice round number of six before long," Jess said.
"In fact, William’s telling everyone that they would love to have one more baby in the next 18 months!"