Kate is thought to have won a private battle with husband Prince William to have another child – and wants to give birth next year.
The Royal couple arrived in Canada for a family trip with son George, three, and daughter Charlotte, 16 months.
Officials are hoping the little prince and princess will woo the public in British Columbia and Yukon during the eight-day tour, boosting the monarchy’s popularity there.
And the visit could provide the perfect chance for Wills and Kate to kick-start their family plans.
A third baby would mark a break from the recent royal tradition of having two children.
Last year Kate was asked at a social event if she would be having a third child soon.
She replied: “I think William would be straight out of the house if I did.”
But last night a close friend of the couple said: “Everyone knows that Catherine would like three children because she comes from a family with a brother and sister.
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“But William didn’t want to rush into it because he has been working hard as an air ambulance pilot and needs his sleep.
Also, it’s kind of an unwritten rule that royals only have two children because Prince Philip has warned for decades that over-population is storing up disaster for the planet.”
They added: “William has held out for several months, but Kate has talked him round and he always puts her wishes first.
“It would be wonderful if they had a third child next year to coincide with the Queen and Prince Philip’s 70th wedding anniversary.
They will be trying as soon as possible – starting in Canada.”
Last night, the royals arrived in Victoria, the capital of British Columbia. George and Charlotte will be based in a nursery at Government House.
Their Spanish nanny will look after them as their parents carry out 32 engagements in eight days.
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Wills and Kate will return to see their children each evening.
But they will spend one night away – and could start trying for their third child in the unlikely setting of a £95-a-night hotel.
They will stay in the Coast High Country Inn in Whitehorse, in the wilds of the Yukon.
Trip Advisor said the hotel has “the smallest rooms we’ve found anywhere in the world” and other guests have complained of yellow water coming from the taps.
But Wills and Kate are said to be looking forward to “roughing it”. They are likely to be given the best room, complete with a giant Jacuzzi and mini-kitchen.
A spokesman for the couple said they were “delighted and honoured” to be making their second visit to Canada, learning about a “country that will play such an important part in their lives for many years to come”.