What if Kate Middleton is expecting twins? Which of the children will end up on the throne?

Some experts are predicting that Kate Middleton and Prince William will end up with twins – and if they do, there could be more than just the usual parenting headaches ahead for the royal couple.

No mother of a future monarch has given birth to twins in Britain since the 15th century – so nobody is quite sure what will happen if Kate Middleton makes history and produces a brace of infants next summer.

And if she does, the intriguing question is: which of the children will end up on the throne?

Normally, of course, this would be little cause for discussion during a royal pregnancy. But the debilitating symptoms currently suffered by Kate Middleton in the King Edward VII Hospital in central London point to hyperemesis gravidarum, a condition experienced slightly more often by women expecting twins.

If it did prove to be the case that Kate Middleton was carrying two children, and she gave birth to them naturally, the rule of primogeniture that has dictated the royal succession for centuries would mean that if the twins are boys, the first-born would ascend the throne.

And until recently, if the twins were boy and girl, it would be the boy who got to wear the crown. But under new legislation, if Kate Middleton gives birth to a mixed pair, it will be the first-born who succeeds – no matter their gender.

All that seems straightforward. But if Kate Middleton were to require either an emergency or an elective caesarean, then things get very much more complicated.

Last year, Crown Princess Mary of Denmark – that country’s next monarch – gave birth to twins.

There was some anxiety beforehand that her babies might have to be delivered by caesarean section, leading to speculation that it would be the obstetrician who chose which child to deliver first and thus which would eventually rise to the throne.

Happily, she didn’t have to go through with a caesarean section birth, and gave birth to Prince Vincent, the nation’s future monarch, 25 minutes ahead of his sister, Princess Josephine.

Were those circumstances to arise here and a caesarean be required, the task of picking who would be the future king or queen would fall, in theory, to Alan Farthing – the royal gynaecologist still better known as the former fiancé of murdered BBC presenter Jill Dando.

Alan Farthing, 47, now a distinguished figure in the world of obstetrics, has held the royal post since 2008, and experienced himself the joy of fatherhood for the first time early last year when his wife Janet, 33, gave birth to a boy.

But while he will be more than prepared for the challenges ahead, it’s unlikely Alan Farthing would want to shoulder the responsibility of choosing whom he brought into the world first. Which begs the question – who would?

Would it be Prince William, who’s expected, like most modern fathers, to be present at the birth? Would the royal family have to revert to the odd practices of yesteryear, where a government minister had to be present at the accouchement to act as an observer and ensure everything was above board?

Last time that happened to a future monarch was in 1926, when the present Queen was born and home secretary Sir William Joynson-Hicks was required to be present, albeit in an adjoining room. Should that be the case again next year the job will fall to Theresa May, the present home secretary.

Faced with the prospect of choosing which twin became the heir, the Cambridges would be able to consider various precedents set when similar situations have arisen.

Their friend the Marquess of Cholmondeley faced such a situation three years ago when his twin sons were born by caesarean. In the end, his ancient title went to his first-born, Alexander, who now bears his father’s courtesy title Earl of Rocksavage; his twin is more simply known as Lord Oliver Cholmondeley.

What Kate and Wills can’t do is draw on any recent experience in either family tree, for there’s almost no history of twins in William’s extended family, and a trawl back through Kate’s rich and varied ancestry comes up with very few examples – and none in recent times.

Should Kate actually prove to be carrying twins, though, she will hope they find better fortune than the last pair of royal infants. In 1430, King James I of Scotland’s wife Queen Joan gave birth to two boys. The first, Alexander, was named as the heir to the throne and given the title Duke of Rothesay – a title still carried today by William’s father as Prince of Wales.

The second twin, James, would have expected an easy life – except that within a year his elder brother had died and he assumed the Rothesay title. By the time he was six, his father had been assassinated and he rose unsteadily to the Scottish throne as King James II of Scotland.

We no longer live in such unsettled times, but for William and Kate, there will still be an element of uncertainty – until they know for certain whether they have twins on the way or not.