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The Evidence on: Due Dates
How do you figure out your estimated due date?
Almost everyone—including doctors, midwives, and online due date calculators—uses Naegele’s rule (listen to the pronunciation here to figure out an estimated due date (EDD).
Naegele’s rule assumes that you had a 28-day menstrual cycle, and that you ovulated exactly on the 14th day of your cycle (Note: some health care providers will adjust your due date for longer or shorter menstrual cycles).
To calculate your EDD according to Naegele’s rule, you add 7 days to the first day of your last period, and then count forward 9 months (or count backwards 3 months).
This is equal to counting forward 280 days from the date of your last period.
For example, if your last menstrual period was on April 4 you would add seven days (April 11) and subtract 3 months = an estimated due date of January 11.
Another way to look at it is to say that your EDD is 40 weeks after the first day of your last period.
In cases where the date of conception is known precisely, such as with in vitro fertilization or fertility tra
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