Naegele's rule and its ACOG override thresholds are checked against the editorial standards page.
Naegele's rule takes the first day of your last menstrual period, adds one year, subtracts three months, and adds seven days, which is arithmetically the same as adding 280 days. That is where every due date on this site starts. What happens after that first estimate, whether it holds or gets replaced by an ultrasound measurement, is spelled out by name in ACOG Committee Opinion No. 700, and that is the part most calculators skip.
The Naegele's rule arithmetic
German obstetrician Franz Naegele described the rule in the 19th century, and the Due Date Calculator on this site runs the modern equivalent: LMP plus 280 days, adjusted if you tell it your average cycle runs longer or shorter than 28 days. If your LMP was March 1, the unadjusted estimate lands on December 5. That is a calendar fact, not a clinical judgment, and you can check it with any date calculator.
What the 280-day count assumes
The rule assumes ovulation on day 14 of a 28-day cycle. Most people's cycles do not match that exactly, and about half of people cannot recall their LMP with confidence, which is one of the reasons ACOG's guidance treats last-period dating as a starting estimate rather than the final word once an ultrasound is available (ACOG Committee Opinion No. 700).
When ACOG says a scan overrules the last-period date
A first-trimester ultrasound measuring crown-rump length is the most accurate dating method available, and the same committee opinion lays out exactly when it should move your due date: more than a 5-day gap before 9 weeks, more than 7 days from 9 weeks through 13 weeks 6 days, and progressively wider windows later in pregnancy as ultrasound dating gets less precise. Below that threshold, the LMP-based date typically stands. Once a first-trimester scan sets your date, providers generally do not revise it again from a later scan, since growth differences by then usually reflect the baby's size rather than an error in dating. See the full table on the editorial standards page.
If your cycle runs long or short
A longer cycle typically means later ovulation, and a shorter one earlier ovulation, which shifts the conception window without changing how weeks are counted from LMP. The Due Date Calculator's cycle-length field adjusts for this directly. If you are unsure of your LMP at all, an ultrasound is the more reliable starting point, prioritizing the earliest available crown-rump length measurement.
A midpoint, not a deadline
The 280-day figure is a median, and normal pregnancies vary around it by weeks, not days; see the accuracy guide for the study behind that. A full-term birth spans 39 weeks 0 days through 40 weeks 6 days under ACOG's own categories, described in the full-term guide. Your own due date and any dating discrepancy are worth confirming with your doctor or midwife, who can weigh your specific chart against the general rule described here.
Check your own date
Enter your last period or conception date and see the Naegele's-rule math applied to your cycle.