Betting odds have reduced from 8-1 to just 4-1 in a shock move.
The bookmaker also has 50-1 odds for Prince Harry and Meghan – now the Duke and Duchess of Sussex – to have triplets.
The news came after Australian singer Missy Higgins let slip a little detail about Meghan's due date.
Missy and Meghan met on Tuesday night at a reception hosted by the Governor-General at Admiralty House.
The Aussie songstress also introduced Meghan to her new bub, Luna.
“What an honour to meet the beautiful Duchess today! She was such a sweetheart. I said I felt sorry for her having to do all these meet-and-greets while 4 months pregnant! What a trooper,” the 35-year-old singer wrote on social media.
Her commented alerted royal watchers to the fact that the Duchess is four months pregnant – fuelling speculation the baby – or babies – will be born in April 2019.
Diaries at the ready!