Royal baby: When to induce?
The birth of the Duke and Duchess of Cambridge's second baby is reported to be overdue, possibly by up to one week. So what could happen now?
Most women go into labour spontaneously by the time they are 42 weeks pregnant.
But for those who don't, research suggests there is a higher risk of stillbirth, so induction is offered to all overdue women.
There are also increased risks that the placenta begins to stop working efficiently and potential problems with oxygen supply to the unborn baby.
The induction process normally starts between 40 and 41 weeks, during ante-natal visits to a midwife.
At that stage, if labour hasn't started, there are ways of helping the process along.
Women can be offered a "membrane sweep", which stimulates the cervix to produce hormones that might trigger natural labour. This involves an internal examination.
But women don't have to have a sweep. Instead, after discussing their options with their midwife, they can wait and set a date for their labour to be formally induced a week or so later at around 10 to 12 days after their due date.
And if labour doesn't start naturally after a sweep, induction is usually the next option anyway although there is also the choice to refuse induction and instead be monitored closely.
Formal induction is a fairly common procedure that means starting labour artificially using drugs. In fact, one in five labours in the UK is induced.
Women have a choice not to be induced, but if pregnancy continues to 42 weeks or beyond, pregnant women and their babies have to be monitored extra carefully to check on their well-being.
Induction is something that the royal couple will have discussed with their midwives and doctors, going through the advantages and disadvantages.
The Royal College of Obstetricians and Gynaecologists says induced labour has an impact "on the birth experience of women".
"It may be less efficient and is usually more painful than spontaneous labour, and epidural analgesia and assisted delivery are more likely to be required."
The NHS Choices websites says women are more likely to ask for an epidural and "should have access to all the pain relief options usually available".
But on the plus side, for women who have given birth before, induction can speed up the birth process considerably.
Dr Daghni Rajasingham, consultant obstetrician at St Thomas's Hospital in London, and spokesperson for the RCOG, says Catherine is likely to experience a much quicker labour if induced this time around: "The womb has a memory and remembers what it's done before."
Women who feel fed up, heavy and uncomfortable if they are overdue are often more than happy to be induced by the time they are into their second week past the due date.
Labours can be induced for other reasons too - if the woman's waters have broken, if the baby is very large or if the woman or her baby have a health problem, such as high blood pressure.
A date is usually set for the procedure in a hospital's maternity unit, which for Catherine would be at the Lindo Wing at St Mary's Hospital in London.
Induction is a safe procedure and involves inserting a tablet or gel to get contractions going.
This can take a while, and some women may be allowed to go home while they wait for it to work.
If contractions still haven't started after six hours, a midwife can artificially break the waters.
Once labour starts, it should proceed normally, although sometimes a hormone drip is needed to speed up labour.