However, by the 1900s, for some unknown reason, American textbooks adopted a form of Naegele’s rule that added 7 days to the first day of the last period (Baskett & Nagele, 2000).Īnd so this brings us to today, where almost all doctors use a form of Naegele’s rule that adds 7 days to the first day of your last period, and then counts forward 9 months-a rule that is not based on any current evidence, and may not have even been intended by Naegele. Most added 7 days to the last day of the last period. His text can be interpreted one of two ways: either you add 7 days to the first day of the last period, or you add 7 days to the last day of the last period.Īs the 1800s went on, different doctors interpreted Naegele’s rule in different ways. (This is how Naegele’s rule got its name!) However, Naegele, like Boerhaave, did not say when you should start counting-from the beginning of the last period, or the last day of the last period. In 1812, a professor from Germany named Carl Naegele quoted Professor Boerhaave, and added some of his own thoughts. However, Boerhaave never explained whether you should add 7 days to the first day of the last period, or to the last day of the last period. Based on the records of 100 pregnant women, Boerhaave figured out the estimated due date by adding 7 days to the last period, and then adding nine months (Baskett & Nagele, 2000). #Due date predictor how toIn 1744, a professor from the Netherlands named Hermann Boerhaave explained how to calculate an estimated due date. This increases the accuracy of the EDD because it no longer assumes a Day 14 ovulation based on the first day of the last menstrual period. In cases where the date of conception is known precisely, such as with in vitro fertilization or fertility tracking where people know their ovulation day, the EDD is calculated by adding 266 days to the date of conception (or subtracting 7 days and adding 9 months). This is equal to counting forward 280 days from the date of your last period.įor 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.Īnother way to look at it is to say that your EDD is 40 weeks after the first day of your last period. 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). 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). How do you figure out your estimated due date?Īlmost 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).
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