Prince William and Kate Middleton's third child's name still remains a mystery, but some believe that the royals could name their baby after a queen.

The current most popular names that the public believes the Duke and Duchess of Cambridge will give their third child include several crowned queens, according to a betting agency. Mary is in the lead with odds 7-1. Meanwhile, Victoria and Elizabeth also made it in the poll with odds of 8-1 and 16-1, respectively.

"We can’t quite put our finger on the link between Mary and the North West, but the overwhelming support means we have a new favourite in the royal baby name market," Jessica Bridge of Ladbrokes Betting Agency said.

"Punters are convinced the Duke and Duchess are expecting another little girl, and obviously it would be a lovely touch to honour Her Majesty's Grandmother, but it will be interesting to see if any boys' names are backed over the coming weeks before the birth," Bridge continued.

Meanwhile, the most popular male name, if Middleton and Prince William end up having a boy, is Albert, with odds 12-1. Other popular choices are Arthur, Frederick, Henry and James.

In January, the most popular choice was Alice. This was followed by Albert as the runner-up. But this has changed. Alice is now in the second place, along with Victoria with 8-1 odds.

Meanwhile, Prince William joked about having twins with Middleton a week ago when the Duke hosted the Centrepoint Awards at the palace. He said he was trying to catch more sleep and having twins would be mentally challenging.

"Our third child is due in April, I'm getting as much sleep as I can," Prince William told Raymond Stoner of Anston Properties.

When Stoner told Middleton's husband that having twins would save time, Prince William gave a surprising and funny response. "Twins? I think my mental health would be tested with twins," the Duke said. "Two is fine — I don't know how I'm going to cope with three, I'm going to be permanently tired."

In related news, Prince William and Middleton are reportedly very strict when it comes to the privacy of their kids. In fact, the Duke and Duchess planted 40-foot hedges in front of Kensington Palace to give Prince George and Princess Charlotte the opportunity to play outside without being seen from the gates.

In addition, when the Cambridges were still living at Anmer Hall, Prince William and Middleton reportedly implemented a "no-fly zone" around their estate. The royal couple asked for it to prevent the helicopters and drones from taking photos of their kids.