It was only a short journey from Kensington Palace to King Edward VII Hospital in central London, but it was the beginning of a long road to reform for the monarchy. Prince William and his wife, Catherine, the Duchess of Cambridge, who spent three nights in hospital with severe morning sickness last week, had the consolation of knowing they were making royal history.
Her morning sickness will wear off, but the decision that their first-born and heir to the throne will take the next place in the order of succession whatever its gender ushers in a new era for Britain and the Commonwealth.
Somewhat belatedly, Britain is moving into line with most other European monarchies. The dynastic change, promoted by Prime Minister David Cameron, has set a conundrum, however, for the Spanish royals. Ever since the monarchy was reinstated there in 1975, the institution has been seen as progressive and modern, endless contrasts made by the Spanish media and public with the backward-looking House of Windsor. With the change in Britain, the pressure is now on Spain to reform its own male-only hereditary principles.
That said, the House of Windsor is moving into line with Europe in other ways. Spain's King Juan Carlos has always argued that he's doing a job for the people and for the institution of monarchy. His son, Prince Felipe, sings from the same hymn sheet. As, for that matter, does Prince William.
Whereas the Queen sees her role as a sacred calling – at her 1953 coronation, an opinion poll revealed that a substantial minority felt the Queen descended directly from God – William believes he can clock on and off as a prince. His decision to sue the French magazine Closer for publishing topless pictures of his wife reveals that he's a young man who believes he's entitled to a private life. Not for him the traditional regal response of "never complain, never explain" when it comes to media intrusion.
Let's not forget that it was William who unwittingly sparked the whole debate and subsequent inquiry about media behaviour in Britain when he discovered that his cellphone was being interfered with by the royal correspondent of the now defunct News of the World.
This is ironic. Unlike his father, Prince Charles, who has successfully used his privilege and position to lobby government ministers on everything from organic farming and modern architecture to complementary medicine, his son shows no inclination to enter the political arena. Apart from supporting England's failed bid to play host to the World Cup, William has stuck firmly to his role as a search and rescue helicopter pilot and cheerleader for the country.
In this, William is proving himself to be in the hands-off tradition of his grandmother and monarchs such as the stamp-collecting George V. By contrast, Charles is more in the mould of Edward VIII, whose famous utterance that "something must be done" regarding jobless miners alarmed the government of the day.
When Charles was 30 – the same age William is now – he was already promoting the Prince's Trust to encourage young people to start their own businesses and chairing meetings between police and disenfranchised minorities. It's hard to see William becoming such a controversial figure. The same, too, can be said of Catherine, whose maiden name reflects her middle-of-the-road inclinations.
Whatever the temperament of the couple's heir, he or she is likely to be a pensioner before they're crowned. Forget the bicycling monarchy; we're moving into an era of the bus-pass monarchy, where the new arrival will have to wait almost a lifetime before inheriting his or her birthright. The maths are inexorable. If the Queen lives as long as the Queen Mother, who died at 101, she will reign until 2027. By then, Charles will be in his late 70s and William middle-aged before he's invested as the next Prince of Wales.
In a society that values youth over experience, the fact that the monarchy is turning – if not fifty – at least several shades of grey may count against its continued popularity in the public imagination. Or, of course, just as London's staging of the Paralympics this year brought disability out of the communal closet, so an ancient royal regime may help remove the cloak of invisibility that comes with age.
One thing is certain. Whether William and Catherine's first-born is a boy or a girl, we can only look forward to a changing of the old guard at Buckingham Palace.
Andrew Morton is the official biographer of the late Diana, Princess of Wales.