The Duke and Duchess of Cambridge have two other children together.
The rules also allowed members of the Royal Family to marry a Roman Catholic and still become king or queen.
This playlist, compiled especially for the couple after news of Kate's pregnancy was shared by Kensington Palace, includes hits from the 1960s to present day - albeit with a female slant.
The Duchess is suffering from hyperemesis gravidarum, a condition of severe morning sickness during pregnancy, according to the Palace.
It was not immediately clear when the baby is due although betting agencies were beginning to offer odds on possible names for the soon-to-be-born child in the House of Windsor.
By having more than two offspring, William and Kate are following in the footsteps of the Queen and the Duke of Edinburgh, who went on to have four children - although there was a gap of 10 years between their second child, Anne, and third, Andrew.
Together with Prince Harry, William and Kate have taken a leading role in marking the 20th anniversary of Diana's death. She is being cared for at their London home.
Kate is the eldest of three siblings, and reportedly had a very happy childhood.
More recently, during a royal tour of Poland, Kate joked about a third child when given a cuddly toy created to soothe tiny babies.
Prince Charles, Queen Elizabeth II's oldest son, is now No. 1 in the succession line.