In a statement Monday, the palace said the royal couple - known as the Duke and Duchess of Cambridge - are very pleased to announce they are expecting a baby, and that members of both their families are delighted with the news.

The statement added that Catherine, 30, is in London's King Edward VII Hospital for treatment of acute morning sickness. It says the pregnancy is in its early stages, and Her Royal Highness is expected to stay in the hospital for several days.

Prince William, Queen Elizabeth's grandson, is second in line to inherit throne after his father, Prince Charles. William's and Catherine's first child will become the third in line when he or she is born.

Last year, Britain and other Commonwealth countries that recognize the British monarch as head of state agreed to new rules that give females equal status with males in the order of royal succession. Previous laws gave priority to male descendants.

William married the former Kate Middleton on April 29, 2011, at Westminster Abbey in a ceremony viewed by an estimated worldwide audience of 2 billion.