What if it's a girl? The constitutional dilemma facing Prince William and a pregnant Kate Middleton

Our diarist on the complications of State that the newest addition to the Windsors will have to confront. Also below: small mercies for the Lib Dems after a recent trouncing

Kate Middleton on a visit with Prince William to Darwen Aldridge Community Academy in Darwen, Lancashire, on their last public outing together before their wedding (PA)

The big constitutional question hanging over Kate Middleton’s belly right now is what if it’s a girl. The short answer is she will escape the fate of other firstborn princesses who had to stand aside and watch their younger brothers succeed to the throne. Technically, the primogeniture rule, which gives all a monarch’s sons precedence over any of his daughters, still applies, but it will be gone long before the unborn child’s great grandmother, grandfather and father have died.

David Cameron secured an agreement at the Commonwealth Conference in Australia last October that all the countries who recognise the Queen as head of state will alter their constitutions simultaneously, a process now being co-ordinated by the New Zealand government. There was some impatience expressed with the slow progress of this procedure when MPs debated it two weeks ago, but we should not blame the New Zealanders.

An already complex issue and is made more so because the Commonwealth governments are simultaneously going to abolish the law which excludes from the line of succession any royal who marries a Roman Catholic. This involves repealing parts of a long list of ancient laws, including the Bill of Rights 1688, the Coronation Oath Act 1688, the Union with Scotland Act 1707, the Princess Sophia's Precedence Act 1711, the Royal Marriages Act 1772, the Union with Ireland Act 1800, the Accession Declaration Act 1910, and the Regency Act 1937.

But the main point is that it will happen, and if the Duchess’s bump is a girl, not only will she follow directly behind her father in the line of succession, but she can marry a Roman Catholic too, if she wishes.

Unlucky day

Lucy Powell, Manchester’s first woman Labour MP, let it be known yesterday morning that she is pregnant - Manchester’s first pregnant Labour MP indeed. In the light of a later announcement, she realised that she had picked a bad day to bury good news.

UnMenschionable

If you were wondering what Conservative MPs were saying on Twitter yesterday about the royal pregnancy, the quick way to find out was to go into a Twitter feed called “Conservative MPs” which collates the tweets of all 127 of them who are on Twitter. I did, and was startled to come upon the thoughts of one Louise Mensch. Apparently, there is a corner of Conservative Central Office where Mrs Mensch is forever an MP.

Pope's surge

Pope Benedict XVI was picking up followers by the tens of thousands on Twitter yesterday before he had written his first tweet and despite the fact that he is not following anyone except himself, in seven languages. Ladbrokes, the bookies were offering odds of 2-1 that he will have a million by 1 January.

Small mercies

A number of commentators, including your diarist, described the Liberal Democrat performance at last week’s Rotherham by-election as the worst ever by any party of government. That is on the basis of their coming eighth, behind a parade of fringe party candidates and assorted oddities.

But curiously, if you measure their achievement by the percentage of total votes cast that went their way, 2.1 per cent, that was actually better than Labour did in a by-election in Winchester in 1997 when they picked up 1.7 per cent of a much bigger turn out. That was a very odd by-election, called because the result had been so close at the general election that a court ordered a re-run, whereupon Labour voters shifted to the Lib Dem to keep the Tory out. No one can be certain that the embarrassingly support shown for the Lib Dems last week was caused by one-off tactical voting, but still, it’s a small thought to cheer Nick Clegg’s beleaguered troops.