For much of the day, the money was on Alexandra for a girl and James or George for a boy. Late Monday, it was announced that Kate had delivered a son, though there was no word on the name.
Betting agency Coral described it as the biggest non-sporting betting event in the company's history, with gamblers racing to wager on everything about the new heir to the throne.
"The whole world has been waiting for Kate to go into labor and now that she has, we have witnessed another betting frenzy," said Nicola McGeady, spokeswoman for the firm, in the hours before the birth was announced.
The betting went well beyond the name.
Paddy Power took bets on the color Kate's sister, Pippa Middleton, will wear when she comes to visit.
Ladbrokes had odds of 33-to-1 that the new heir, who would be third in line for the throne, would represent Great Britain at the Olympics. It's not that far-fetched equestrian athlete Zara Phillips, the queen's granddaughter, won silver at the 2012 London Olympics.
The public may have to wait, though, to cash in their betting slips. It is not uncommon for royals to take their time naming babies: Queen Elizabeth II and her husband Prince Philip waited for a month in the case of Prince Charles.