Your Due Date Comes From a Rule Written in 1806. Here Is Why It Still Works.
The date circled on a printout from a midwife's office will shape the next nine months of a pregnant woman's life. Employers need to know when parental leave begins. Airlines have cutoff windows. Hospital birth centers ask expectant parents to call when contractions reach a certain frequency. Induction conversations start when the calendar passes forty weeks. A single date, derived in a few seconds from a smartphone app or a paper wheel, carries enormous practical weight.
That date comes from arithmetic developed in Heidelberg around 1806 by a German obstetrician named Franz Karl Naegele. He was born in Dusseldorf in 1778, trained in the medicine of his era, and in 1806 took a position as professor and director of the lying-in hospital at Heidelberg University. It was from that hospital, attending to laboring women and tracking their pregnancies in meticulous records, that he developed the observation he would later codify into a rule that now runs inside every due date app and hospital intake form in the world.
The remarkable thing is not that the rule has survived two centuries. The remarkable thing is that modern molecular research, conducted with tools Naegele could not have imagined, has largely confirmed his original estimate. This article explains how he arrived at 280 days, what has been refined since, and exactly when the rule breaks down.
Why 280 Days Has Held Up for Two Centuries
Naegele's observation was that human pregnancies tend to last approximately ten lunar months from the last menstrual period. He formalized this into a simple arithmetic rule: take the first day of the last menstrual period, add nine months and seven days. The result is an estimated due date at 280 days from LMP, or exactly 40 weeks. The calculation was designed to be done with nothing more than a calendar, and that simplicity was part of its appeal in an era before medical instruments could image the developing fetus.
His textbook "Lehrbuch der Geburtshilfe," first published in 1830 for use by midwives, went through fourteen editions. The rule it described spread through obstetric practice across Europe and eventually worldwide. By the time ultrasound arrived in obstetrics in the mid-20th century, Naegele's formula had already been embedded in clinical training for over a hundred years.
What makes the rule scientifically credible is that population data keeps supporting it for women with regular cycles. A 2013 study published in Human Reproduction by Jukic and colleagues tracked 125 women whose exact ovulation dates were confirmed through hormone measurements, not estimated. The median time from ovulation to delivery in that cohort was 268 days. Since ovulation in a standard 28-day cycle occurs approximately 14 days after the start of the menstrual period, 268 days from ovulation maps to 282 days from LMP. That is within two days of Naegele's 280-day estimate, a margin that falls well inside natural biological variation. The study also found that the total variation in gestational length among uncomplicated pregnancies spanned 37 days, even when cycles were regular and conceptions were precisely timed. The due date, in other words, was always an estimate. Naegele understood this, too. He published it as a guide, not a guarantee.
When the Rule Gets It Wrong
Naegele's formula rests on one assumption that breaks down for a large fraction of the population: it requires a 28-day menstrual cycle with ovulation on exactly day 14. When either of those conditions is not met, the rule's output drifts from reality by a predictable amount.
A woman with a 35-day cycle ovulates around day 21, not day 14. Her actual conception is seven days later than Naegele's formula assumes. If her due date is calculated straight from LMP without adjusting for cycle length, the estimate will be seven days early, meaning the formula will flag her pregnancy as "late" when she is actually right on time. This matters because approaching 41 or 42 weeks is when induction conversations begin. A wrongly calculated date can put pressure on a pregnancy that is developing normally.
The fix is straightforward: for each day a cycle exceeds 28, the due date moves forward by one day. A 35-day cycle adds 7 days to the standard calculation. A 21-day cycle subtracts 7. Most clinical providers apply this adjustment automatically when cycle length is known, but it requires the patient to know and report her cycle length accurately, which is not always the case.
Irregular cycles add a different problem. When the interval between periods varies substantially from month to month, even a cycle-length adjustment cannot correct the estimate well because there is no reliable baseline to adjust from. About 20 to 30 percent of women have cycles that vary by more than a week from month to month. For this group, LMP-based dating has a meaningful error rate from the start.
How Ultrasound Refined the Rule Without Replacing It
The introduction of ultrasound into routine prenatal care in the latter half of the 20th century gave clinicians a second way to estimate gestational age, one that does not depend on the woman's memory of her last period or assumptions about her cycle. Crown-rump length measurement in the first trimester, typically done between 8 and 13 weeks, can date a pregnancy to within plus or minus 5 to 7 days under normal conditions, compared to a margin of 7 to 14 days for LMP-based dating alone.
ACOG guidelines, updated in 2017, specify that when first-trimester ultrasound and LMP dating disagree by more than 7 days, the ultrasound measurement takes precedence in setting the official estimated due date. A study examining this protocol found that 40 percent of women who received first-trimester ultrasound had their due dates adjusted as a result. That is not a small finding. It means that for roughly four in ten pregnancies, Naegele's rule, applied to the patient's stated LMP, was off by enough to matter clinically.
This does not make Naegele's rule wrong. It means the rule works well when its assumptions hold, and first-trimester ultrasound is the tool for identifying the cases where those assumptions do not. The two methods are complementary. LMP-based dating gives an immediate estimate from the first prenatal visit. Ultrasound either confirms it or corrects it before the second trimester.
Conclusion
Only 4 to 5 percent of babies are born on their calculated due date. A pregnancy is generally considered full-term between 39 and 40 weeks, and most deliveries fall within a two-week window centered on the estimated date. The 2013 Jukic study reinforced just how wide the natural distribution is: even in a cohort of precisely timed, uncomplicated pregnancies, gestational length varied by over five weeks.
The practical implication is that a due date is better understood as the center of a distribution than as a deadline. Arriving two weeks early or two weeks late is within the normal range for a healthy pregnancy. The date still matters, because it sets the reference point for tracking fetal development, scheduling standard screening windows, and deciding when intervention is appropriate. But treating it as a precise prediction rather than a statistical midpoint creates unnecessary anxiety when the baby has not appeared by the circled date on the calendar.
A pregnancy due date calculator gives you Naegele's calculation directly, adjusted for your cycle length if you enter it. The number it returns is the same estimate that obstetricians have been writing on charts since the early 1800s, validated by two centuries of delivery records and confirmed, within a few days, by modern molecular research. Use it as a planning anchor. Expect the baby to have other ideas.
Frequently Asked Questions
What is Naegele's rule?
Naegele's rule calculates a due date by adding 280 days (40 weeks) to the first day of the last menstrual period. It was published by German obstetrician Franz Karl Naegele around 1806.
How accurate is the 40-week pregnancy due date?
Only 4 to 5 percent of births happen on the exact due date. Most deliveries fall within a two-week window around the date. First-trimester ultrasound improves accuracy when cycle length is irregular.
Does cycle length affect your due date?
Yes. Naegele's rule assumes a 28-day cycle with ovulation on day 14. If your cycle is longer, your actual due date is later than the calculated one. Most providers adjust when cycle length is known.