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The estimated due date is calculated by Naegele's rule as the first day of the last menstrual period plus 280 days, which is 40 weeks. The count deliberately begins at menstruation rather than conception because the LMP is a date most people can recall while the moment of fertilization is not observable. As a consequence gestational age runs roughly two weeks ahead of embryonic age: at about 4 weeks gestational, when a home test first turns positive, the embryo itself is only around 2 weeks old. The rule assumes a 28-day cycle with ovulation on day 14, so a cycle of length L shifts the estimate by L minus 28 days, pushing the date a week later for a 35-day cycle and a week earlier for a 21-day cycle. The result is a distribution rather than an appointment. Only about 4 percent of births occur on the predicted day, and roughly two-thirds fall within a week on either side of it. The American College of Obstetricians and Gynecologists defines 39 0/7 to 40 6/7 weeks as full term, 37 0/7 to 38 6/7 as early term, 41 0/7 to 41 6/7 as late term, and 42 0/7 onward as postterm. Irregular or anovulatory cycles, recent hormonal contraception, and implantation bleeding mistaken for a period all degrade LMP-based dating, which is why a first-trimester ultrasound crown-rump length measurement, accurate to about 5 to 7 days, takes precedence when the two methods disagree beyond a set margin.

The 280-day rule carries the name of Franz Karl Naegele, professor of obstetrics at Heidelberg, who set it out in the early nineteenth century and popularized it through his midwifery textbook of 1830; the same arithmetic had been described earlier, and the Dutch physician Herman Boerhaave is often credited with stating it around 1744. Naegele's version was a calendar shortcut for an era in which no one could observe a pregnancy directly: take the first day of the last period, add seven days, subtract three months, and adjust the year. It endured because it required nothing but a calendar and produced a consistent, checkable answer. Direct measurement arrived with obstetric ultrasound. Ian Donald and colleagues in Glasgow reported diagnostic use of pulsed ultrasound in The Lancet in 1958, and by the mid-1970s crown-rump length charts, notably those of Robinson and Fleming published in 1975, allowed gestational age to be estimated from fetal size during the first trimester. Current practice combines both methods rather than replacing one with the other: a 2017 committee opinion from the American College of Obstetricians and Gynecologists, the Society for Maternal-Fetal Medicine, and the American Institute of Ultrasound in Medicine specifies when an ultrasound date should override an LMP date. The vocabulary was tightened separately; the term definitions in use today came from a 2013 workgroup convened under the ACOG reVITALize initiative, which ended the practice of calling anything from 37 weeks onward simply term.