Naegele's rule

Naegele's Rule is a standard way of calculating the due date for a pregnancy. It is named after Franz Karl Naegele (1778-1851), the German obstetrician who devised the rule.

The rule works by subtracting three months from the first day of the woman's last menstrual period and adding seven days to that date. The rule effectively states that a human pregnancy should last approximately 40 weeks (280 days) from the last menstrual period (LMP), or 38 weeks (266 days) from the date of conception.

This method is a good estimation but can be inaccurate for women with irregular menses or confusion about when their last period was. This estimation can easily be a week or more off, it should be used to calculate a range of due dates and confirmed with ultrasound in the first trimester.