Naegele's rule

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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.

Franz Karl Naegele was born July 12, 1778 in Duesseldorf, Germany. In 1806 Naegele became ordinary professor and director of the lying-in hospital in Heidelberg. His "Lehrbuch der Geburtshuelfe" published in 1830 for midwives enjoyed a successful 14 editions.

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 fairly good estimation of the due date. But it is based on ovulation and fertilization happening on cycle day 14. It is less accurate when ovulation happens earlier or later in the cycle, and for women with irregular menstrual periods. Sometimes women may have a light period in early pregnancy, which adds to this confusion. Since 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. The most accurate method for determining gestational age is early ultrasound exam, which is in general use. If the pregnancy is too far advanced for this, or for some other reason one is unavailable, Naegele's rule is a good estimate.