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How Accurate are Due Dates?

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The anticipation of knowing exactly when your baby will be born is exciting. When a physician determines your due date, however, it's not always a guarantee of the day your child will be born. Due dates are determined based on a number of factors, including your menstrual cycles and the baby's measurements, which can predict your baby's expected date of delivery within a week or two.

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Evaluating Your Cycles

Due dates are accurate if you have regular monthly periods; obstetricians often calculate the due date based on the first day of a woman's last menstrual period, says Dr. Draion Burch, Pennsylvania-based obstetrician-gynecologist. However, women who have irregular periods can have inaccurate dates determined, he says. Burch recommends confirming due dates within the first trimester to improve the accuracy of the due date.

Analyzing Baby's Size

Due dates are also calculated based on the baby's body length during an ultrasound between eight and 12 weeks of pregnancy, according to Dr. Sachchidananda Maiti, England-based obstetrician and gynecologist. This method is more accurate than calculating through menstrual dates, he says, but it is not as good at dating your due date if you had assisted conception with fertility drugs and treatments. Doctors usually give women a range between 38 and 42 weeks of a pregnancy as the anticipated delivery time.

Deciphering Accuracy

Due dates are often referred to as estimated due dates because a woman's cycle and the baby's growth can vary, says Deena Blumenfeld, Pennsylvania-based Lamaze certified childbirth educator. "Only 5 percent of women give birth on their due dates, so we often tell moms that it is more like a due months," she says. Although premature delivery is possible, most women deliver between 37 and 42 weeks. Women aren't officially overdue until after 42 weeks of pregnancy, says Blumenfeld.

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Measuring Baby's Growth

Although your due date may be determined based on an ultrasound or your menstrual cycle, a margin of error with the date's accuracy is possible because of the baby's growth, says Blumenfeld. Babies grow and develop at different rates in the womb, just as they do on the outside, she says. "There is a range of normal for fetal development and that affects when labor begins," says Blumenfeld.

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