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20The (In)Accuracy Of Due Dates

Fit Pregnancy and Baby

Sometimes the due date can be off by days or even weeks, and it all has to do with the way the due date is calculated. Doctors usually use Naegele's Rule, which estimates the due date in part based on the average length of a menstrual cycle–28 days, according to

New York Magazine. Women's cycles can vary as much as a week, give or take. Naegele's Rule is also based on knowing the first day of the pregnant mama's last period, and many women don't keep track. Other factors like irregular ovulation can skew the due date. Only about five percent of women actually give birth on their due date.

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