Prince William and Kate, Duchess of Cambridge, expecting 3rd child

PHOTOGRAPHER: The New York Times

Prince William and his wife, Kate, the Duchess of Cambridge, in December 2014.

LONDON — Prince William and Catherine, the Duchess of Cambridge, are expecting a third child, who would be fifth in line to the British throne, Kensington Palace announced Monday.

The announcement came four days after the 20th anniversary of the death of William’s mother, Diana, Princess of Wales, in a Paris car crash, an event that jolted — but also helped to modernize — the monarchy.

William and Catherine, known as Kate, in many ways have — or are least have projected — a more modern relationship than did their predecessors. Both 35, they met as undergraduates at the University of St. Andrews in Scotland and dated for many years before marrying, in contrast with Diana, who had seen Prince Charles, the Prince of Wales, only 13 times before their marriage in 1981.

While not the first commoner to marry a royal, Kate has distinctly middle-class roots; her parents started a successful mail-order company for party supplies.

William and Kate married on April 29, 2011. Their first child, Prince George, was born on July 22, 2013, and their second, Princess Charlotte, on May 2, 2015.

As in her previous pregnancies, the palace announced, Kate has hyperemesis gravidarum, or extreme, persistent nausea and vomiting, which can lead to dehydration, weight loss and electrolyte imbalances. It is more serious than morning sickness, the mild nausea and vomiting that occurs in early pregnancy.

Kate canceled a planned appearance at Hornsey Road Children’s Centerin the Islington neighborhood of London on Monday and is being cared for at Kensington Palace. The palace is the residence of the couple and of William’s younger brother, Prince Harry.

Queen Elizabeth II, who acceded to the throne in 1952 and is the longest-reigning monarch in British history, has four children, eight grandchildren and five great-grandchildren.

The new child would be fifth in line to the throne, after Charles, William, George and Charlotte, and before Harry. Under changes adopted in 2011 by Britain and other Commonwealth countries that recognize the British monarch as their head of state, male heirs no longer receive precedence over their older sisters — a tradition known as primogeniture — in the line of succession.

Elizabeth was the elder of two daughters of King George VI, who had no sons. On Monday, the queen was in Scotland with her husband, Prince Philip, the Duke of Edinburgh, to open a new bridge over the Firth of Forth.

The news was shared widely on social media, with numerous messages of congratulations for the family but also some grumbling about the monarchy. Several commentators sarcastically pointed out that the government recently limited certain welfare benefits for low-income or jobless parents with more than two children — a move that experts say could push 200,000 children into poverty.