Kate Middleton’s baby boy was overdue, royal sources have confirmed.

What does this mean for the birth?

Fewer than one in 20 women gives birth on their “due date” – the expected date of delivery given at the start of pregnancy.

Around half of pregnant women go into labor up to two weeks later, or labor is medically induced during this period because of worries about the health of mother and baby.

Prof. Andrew Shennan, an internationally renowned specialist, says it is more accurate to talk about a “due month” of birth because each woman’s circumstances are different.

Andrew Shennan, who works as professor of obstetrics based at St Thomas’ Hospital, London, said his unit’s policy is not to intervene until a woman reaches 42 weeks of pregnancy – two weeks overdue – unless there is cause for concern.

It is thought that Kate Middleton was 41 weeks pregnant today and comfortably within the normal margin for giving birth.

Prof. Andrew Shennan said: “Most women want to give birth naturally when possible and if there is no reason to induce, we’d rather not because it involves using drugs.

“Only about four per cent of women deliver on their due date, it’s better to talk about a due month, because when they are giving birth naturally around half will deliver later.”

Kate Middleton’s baby boy was overdue

The placenta – the baby’s support system in the womb – is equipped to last for about 40 to 42 weeks which means if labor hasn’t started there is a risk of the baby running out of nutrients at a time when it has never been bigger and more needy.

Another reason for intervening earlier is when there are signs of pre-eclampsia, which raises blood pressure and can threaten the lives of mother and baby.

When a decision is taken to induce labor, a number of drugs may be used to soften up the cervix – the neck of the womb – and start contractions including prostaglandin pessaries and oxytocin through a drip.

Obstetricians take a stepwise approach and if something works, there’s no need to continue with another drug, said Prof. Andrew Shennan.

These are normally given in hospital because the baby’s vital signs should be monitored throughout, he added.

Like thousands of other first-time mothers, Kate Middleton was asked to keep track of her contractions, the early signs of labor, at home.

She would have had instructions to contact her medical team when they were strong and regular, coming at the rate of two or three every 10 minutes, or if she was worried at any time.

Contractions are the tightening of muscles in the womb to push the baby down.

Prof. Andrew Shennan said: “It’s OK to stay at home at this stage and there is some evidence that women do better as a result.

“When they increase to three or four every 10 minutes then normal labor has started and she will need to set off for the hospital, allowing for time of day and traffic conditions.

“A lot depends on the level of discomfort and the woman’s pain threshold.”

The early stages of labor – which led to Kate Middleton’s admission to hospital around 5.30 a.m. this morning – may last some time, but when contractions speed up and the labor becomes established the timeline is carefully monitored.

The neck of the womb dilates, or widens, by one centimetre an hour and needs to get to 10 centimetres.

Often the second half of labor is signaled by the waters breaking – when the sac of amniotic fluid which protected the baby during pregnancy begins to leak.

Prof. Andrew Shennan, a leading researcher for the baby charity Tommy’s, said the early stages of labor can “niggle” for days.

“But when labor is established it’s a much clearer picture and ideally there will be good progress over 12 hours.

“Any longer and we would be considering a Caesarean section to ensure the safety of mother and child,” he added.