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Why are some babies overdue?

By Ashita Ganguly, Special to CNN
updated 9:24 AM EDT, Thu July 18, 2013
STORY HIGHLIGHTS
  • First born babies are less likely to arrive on time
  • Mothers may also be more likely to deliver late if overdue pregnancies run in the family
  • Excitement about the royal baby's arrival has been growing

(CNN) -- The world is eagerly waiting to hear news of the birth of the royal baby. A royal source told CNN that the due date was Saturday, July 13, which would mean that the Duchess of Cambridge is several days overdue.

Why is it taking the future prince or princess of Cambridge so long to arrive?

First-born babies are less likely to arrive on time -- they're either too early or too late, studies have found. A normal pregnancy lasts about 40 weeks, according to the American College of Obstetricians and Gynecologists. After 42 weeks, the pregnancy is considered to be post term. However, doctors usually induce labor before the pregnancy reaches this stage.

Royal babies: Five things you didn't know

Doctors predict the due date by noting the first day of the mother's last period, as well as measuring the size of her uterus early on in the pregnancy. However, differences in menstrual cycles and inaccuracies in remembering the actual date of the mother's last period make predicting a due date more of an art than a science.

"Often, there is a miscalculation of the due date in the first place," Dr. Neil Sloane, clinical director for obstetrics and gynecology at Delaware Valley Health in Philadelphia, noted. "And when we examine a supposedly overdue baby, they don't look like they're overdue."

Mothers may also be more likely to deliver late if overdue pregnancies run in the family. "Family history of overdue pregnancies is a stronger predictor than whether or not it is a first-time mother," said Dr. Daniela Carusi, director of general gynecology and surgical obstetrics at Brigham and Women's Hospital in Boston.

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She added that "People who have the most accurate due dates are those that have IVF, because they know exactly when the baby was conceived."

As pregnancies progress beyond the due date, both mother and baby could face health risks as the placenta ages and the amount of amniotic fluid decreases. Both of these events could possibly lead to the baby receiving less oxygen and nutrients.

Excitement about the baby's arrival has been growing ever since Kate was hospitalized for morning sickness, with royals and non-royals alike speculating on names and gender.

Royal baby birth: Everything you ever wanted to know

The waiting game continues as the world watches outside the gates of Buckingham Palace until royal officials announce the birth of the next heir to the throne.

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