William, Queen Elizabeth's grandson, is second in line to the British throne, and their first child will become the third in succession when he or she is born.
Last year Britain and other Commonwealth countries which have the queen as their monarch agreed to change the rules of royal succession so that sons would no longer have precedence as heir, regardless of age.
The agreement also means an end to a ban on a future monarch marrying a Catholic, a stipulation dating back some 300 years.
William and Catherine currently live in north Wales where the prince is based as a Royal Air Force search and rescue pilot.
