The frenzy was sparked when William was given a babygrow by a wellwisher in Cambridge on Wednesday.
Kate has changed to a more mature-looking fringe cut which also fuelled speculation she is getting ready for the next step.
“We saw numerous bets for a royal new arrival throughout the Jubilee summer,” the Daily Star quoted bookmaker Coral’s David Stevens as saying.
“But the betting went through the roof again this week. We cut the odds to 1-4 and then again to 1-8, at which point we decided to suspend the betting.
“If the money talks, 2013 will see a new addition to the Royal Family,” he said.
Ladbrokes have cut the odds to 1-5, while the odds on twins are 33-1.