According to YourDueDate.com,
Your due date is calculated by adding 280 days (40 weeks) to the first day of your last menstrual period (assuming a 28 day cycle).
Note that your menstrual period and ovulation are counted as the first two weeks of pregnancy. If you deliver on your due date, your baby is actually only 38 weeks old, not 40.
According to BabyCentre,
Unless you’ve been tracking your ovulation, there’s no way to know for sure exactly when you ovulated and conceived. That’s why your due date is calculated based on the date of your last period instead.
Nickzom calculator is capable of estimating your due date for you in a matter of seconds. All you need to know to get your due date are:
- The first day of your last period
- Your average length of cycles
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