Prince William and Kate Middleton
Kate Middleton and Prince William broke royals' sex tradition. Pictured: Prince William and Middleton arrive at a reception at Claerchen's Ballhaus dance hall following a day in Heidelberg on the second day of the royal visit to Germany on July 20, 2017 in Berlin, Germany. Getty Images/Sean Gallup

Prince William and Kate Middleton broke a royal tradition by not having a child on the first year of their marriage.

The Duke and Duchess of Cambridge are already parents to 4-year-old Prince George and 2-year-old Princess Charlotte. However, it took over two years after they exchanged vows before they welcomed their first child, which was unexpected from a royal.

"By the time the Duke and Duchess of Cambridge celebrated their first wedding anniversary, they were the first future monarchs in 200 years not to have conceived within the first year of marriage," royal author Gill Knappett said.

According to Knappett, there was a reason why Prince William and Middleton did not have a child immediately. This was reportedly due to the Duke's commitment to Queen Elizabeth II which "he has been preparing for his whole life and readily accepts" and which the Duchess "equally accepts in her role as wife of a future King of England."

"It is thought that William and Catherine, as newly-weds, may have delayed staring a family so that they could participate fully in the Queen’s Diamond Jubilee celebrations in 2012," Knappett continued. "Part of William and Catherine’s commitment to the Queen in her Diamond Jubilee year involved the young couple touring Asia on behalf of Her Majesty in September 2012."

During the engagement, Prince William and Middleton visited Singapore, Malaysia, Kuala Lumpur and Sabah. They also flew to the Solomon Islands and Tuvalu in the South Pacific.

Prince William and Middleton are set to welcome their third child on April 23. Middleton will give birth in a luxury suite at the Lindo Wing at St. Mary's Hospital in Paddington. The Duchess delivered Prince George and Princess Charlotte in the same facility in 2013 and 2015, respectively.

Middleton's room, which reportedly costs as much as $9,650 per day, has been prepared and secured. In fact, it has been on lockdown since March, and the security does regular sweeps in the area.

"Security does regular sweeps of the room, then it's resealed each time with tamper-proof tape. It's a high-security operation. Even the crawl space above the room is checked regularly," a source said.

The hospital has been preparing for Middleton's delivery for weeks now. They repainted the railings outside the Lindo Wing and also placed a "No Parking" sign in the area.