How is the due date calculated?
The Naegele rule and its medical significance
The Naegele rule is the most widely used method worldwide for calculating the expected date of birth. It was described in 1812 by the German obstetrician Franz Karl Naegele and remains the standard procedure in obstetrics to this day. The formula adds 280 days (40 weeks of pregnancy) to the first day of the last menstrual period (LMP).
Our calculator extends the classic Naegele rule with a cycle correction: since the original formula assumes a standard 28-day cycle, the calculated date shifts accordingly for a deviating cycle length. For a 30-day cycle, the calculator computes EDD = LMP + 282 days (+2 days), and for a 26-day cycle EDD = LMP + 278 days (−2 days). This correction significantly improves the accuracy of the estimate.
It is important to understand that the calculated due date is a statistical estimate. In fact, only about 5 percent of all babies are born exactly on the due date. Most births occur in the period from one week before to two weeks after the calculated date. An ultrasound examination in the first trimester (usually between weeks 9 and 12 of pregnancy) allows for a significantly more precise determination of the date by measuring the embryo's crown-rump length.
In Germany, the maternity guidelines (Mutterschaftsrichtlinien) require at least three ultrasound examinations during pregnancy. The first screening serves, among other things, to confirm or correct the calculated due date. In the event of a significant discrepancy between the calculated EDD (Naegele) and the sonographic EDD, doctors correct the date based on the ultrasound measurement, as this is more precise in early pregnancy.