Much excitement but less constitutional weight

Media crews at Kensington Palace Image copyright AP

For a second time, William and Kate have been forced to announce a pregnancy before the duchess passed the significant 12-week milestone.

For a second time it's because Kate is suffering from an acute form of morning sickness - though this time she's being treated behind palace walls and not at a private hospital with representatives of the world's media gathered outside.

The pregnancy has generated international excitement and will continue to do so. But this impending birth will lack the constitutional significance of Prince George's arrival.

As things stand, the as yet unborn girl or boy isn't destined to be a monarch. S/he will occupy the same position as Prince Harry once did.

His mother, Diana, used to call Harry "the back up". Today's announcement has performed one unintended but useful role for the Windsors.

Headlines about the Queen's view of Scottish independence will be replaced by extensive coverage of a royal birth next year.