Where's that royal baby? Is Kate overdue?

By the time you're reading this, the Duchess of Cambridge could be in labor. Or it could be a matter of hours. Or days. Or weeks.

As Britain's Prince William and his wife, Kate, await the birth of their first child — and the future heir to the English throne — some are convinced the royal due date has already passed, even though Buckingham Palace has not given an exact date. 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 the Duchess is due to deliver the baby in "mid-July."

Catherine, Duchess of Cambridge (Photo by Getty Images)

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.

Catherine, Duchess of Cambridge

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. "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 3 to 12 percent of women in the U.K. go beyond that term. After 41 to 42 weeks, doctors may consider inducing labor. Doctors or midwives typically induce labor with medications or other methods; it may still take one to two days for contractions to start.

Catherine, The Duchess Of Cambridge meets children and staff during a visit to Naomi House Children's Hospice. (Photo by Getty Images)

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

Catherine, Duchess of Cambridge

In Britain, about 25 percent of all births are cesarean sections, versus 30 percent 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.

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 labor. Pregnant women are also advised to keep as active as possible even after their due date passes.

Prince William, Duke of Cambridge (L) and Britain's Catherine, Duchess of Cambridge

Rajasingam also said women having their first baby, as in the case of the Duchess, 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.

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