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, George, was born on July 22, 2013, and their second, 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 Center in 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 Prince 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.