Maybe it's only a matter of hours before the Duchess of Cambridge is in labour. Or days. Or weeks.

As Prince William and his wife, Kate, await the birth of their first child — and the future third in line to the throne — some are convinced the royal due date has already passed, even though Buckingham Palace has not given an exact date.

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Royal supporter Margaret Tyler has balloons ready outside the Lindo Wing of St. Mary's hospital, where the Duchess of Cambridge is due to give birth any day. (Kirsty Wigglesworth/Associated Press)

Many in the British media predicted the baby would be born last week and the prince himself is now on official leave. Dozens of reporters have already staked out the central London hospital where Kate is expected to give birth. The palace has said only that she is due to deliver the baby in "mid-July."

But experts say there's no reason to think that the baby is actually overdue; due dates are at best an educated guess and come with a margin of error of two to three weeks.  

"The baby will come when he or she is ready," said Janet Fyle, a midwife and professional policy adviser at Britain's Royal College of Midwives. She said the due date is calculated from the first day of the woman's last period. Then add seven days, plus nine months.

"But nature is the primary determinant [of the due date] and we can't do anything to change that," Fyle said.

An ultrasound done around weeks 11 to 12 of the pregnancy can also give women a better idea of when exactly to get the nursery ready, she said. A head measurement at that point is a better indicator of age than later in pregnancy.

For healthy pregnant women, as the Duchess of Cambridge appears to be, Fyle said they shouldn't worry if they haven't given birth by their predicted due date. Many things can delay birth, including the baby's head being in the wrong spot.

Getting ready

"It may just take the baby awhile to get into the right position before he or she is ready to come out," she said.

Normal pregnancies last about 40 weeks, though three to 12 per cent of women in the U.K. go beyond that term. After 41 to 42 weeks, doctors may consider inducing labour. Doctors or midwives typically induce labour with medications or other methods; it may still take one to two days for contractions to start.

"There is a risk as the pregnancy goes on that the placenta may not work as well," said Dr. Daghni Rajasingam, a spokeswoman for Britain's Royal College of Obstetricians and Gynecologists.

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Prince William and Kate leaves the King Edward VII hospital in central London on Dec. 6, 2012, after she was treated for a severe form of morning sickness in the early stages of her pregnancy. (Alastair Grant/Associated Press)

She said that could compromise the baby's growth and oxygen supply. For women who don't want to have an induced labour, even if they are beyond 40 weeks, Rajasingam said extra scans to monitor the baby's heartbeat and movement would be necessary.

In Britain, about 25 per cent of all births are cesarean sections, versus 27 per cent in Canada and 30 per cent in the U.S.

In 2011, Britain's health watchdog decided women should be able to get a C-section on demand under the free health-care service, though there is no indication whether the Duchess of Cambridge has requested one in the private wing of the hospital where she is expected to deliver.

Keep active

Although the Duchess of Cambridge was hospitalized last December for severe morning sickness, Rajasingam said there's no reason to think she might have a more difficult labour. Pregnant women are also advised to keep as active as possible even after their due date passes.

Rajasingam also said women having their first baby, as in Kate's case, typically have a longer pregnancy than with subsequent children, though scientists aren't exactly sure why.

"It's like a racehorse on a track," she said. "For a horse that hasn't run the race before, it can take a little bit longer if you're not used to it," she said.