How this instrument works
Naegele's rule estimates a due date as 280 days — 40 weeks — from the first day of the last menstrual period (LMP). It is a convention, not a measurement: it assumes a 28-day cycle with ovulation on day 14, so it counts forward from a date roughly two weeks before conception actually happens. This instrument works from a day count rather than a calendar field — enter how many days have passed since the LMP, and it returns how many days remain to the 280-day mark.
The rule dates to the German obstetrician Franz Naegele in the early 1800s, and it has stuck around because it is simple and needs only one remembered date. The American College of Obstetricians and Gynecologists (ACOG) still treats the 280-day, LMP-based convention as the default for assigning a due date, while recommending first-trimester ultrasound dating whenever the LMP is uncertain or the two disagree by more than about a week.
Treat the result as a statistical average, not a deadline. Only a small minority of people deliver on their exact due date — ACOG notes it is roughly 1 in 20 — and full-term birth routinely arrives anywhere from about two weeks before to two weeks after this estimate. Cycle length, ovulation timing, and ordinary biological variation all move the real delivery date away from the arithmetic.
- Count the number of days that have passed since the first day of your last menstrual period.
- Enter that figure in Days Since LMP — this field takes a day count, not a calendar date.
- Read Days Until Due — the number of days remaining to the 280-day (40-week) estimate.
- Recalculate any time; if your LMP shifts or an ultrasound gives a different figure, update the day count and the estimate updates with it.
Worked example — 90 days since LMP
Ninety days have passed since the first day of the last menstrual period. The rule subtracts that from the 280-day constant: 280 − 90 = 190. That leaves 190 days — about 27 weeks — until the estimated due date.
The same subtraction explains the edges. On day 0, the day of the LMP itself, the full term is still ahead: 280 − 0 = 280 days remain. At day 280, the countdown reaches zero and the estimated due date has arrived — though real deliveries routinely land anywhere from roughly two weeks earlier to two weeks later, since this is a population average rather than a fixed schedule.
Questions
What is Naegele's rule?
Naegele's rule is the standard convention for estimating a pregnancy due date: add 280 days (40 weeks) to the first day of the last menstrual period. It assumes an average 28-day cycle with ovulation around day 14, so the count starts about two weeks before conception actually occurs. It has been the default obstetric dating method since the early 1800s and remains the basis ACOG uses today, alongside ultrasound confirmation.
Why does this calculator ask for a day count instead of a date?
This site's date arithmetic runs on day counts rather than calendar dates, so instead of entering a calendar date for your last period, you enter how many days have already passed since it. The underlying rule is unchanged — 280 days from the first day of the LMP — the only difference is that you work out the initial subtraction, today minus your LMP date, before typing in the number.
How accurate is the 280-day estimate?
It is a population average, not a precise forecast. ACOG notes only about 1 in 20 people deliver on their exact estimated due date, and full-term deliveries routinely happen anywhere from roughly two weeks before to two weeks after the 280-day mark. Ultrasound measurement in the first trimester is generally considered more precise than LMP dating alone and is recommended when the two disagree meaningfully.
Why 280 days and not a rounder number?
280 days is exactly 40 weeks, the traditional obstetric estimate of full gestation counted from the LMP rather than from conception. Counted from actual conception, average gestation runs closer to 266 days — 38 weeks — which is why this site's EDD-from-conception calculator uses a different constant for anyone who knows their conception date directly, such as from an IVF transfer.
What if my menstrual cycles are not 28 days long?
Naegele's rule assumes a 28-day cycle, so if yours run consistently longer or shorter, the 280-day estimate can drift away from your actual ovulation timing. Clinicians often adjust for a known, atypical cycle length, and first-trimester ultrasound dating is generally treated as more reliable than LMP-based dating alone for anyone with irregular cycles.
Will my doctor use this exact number?
Possibly not exactly. Clinicians typically confirm or adjust an LMP-based estimate with a first-trimester ultrasound, and ACOG recommends using the ultrasound-derived date when it differs from the LMP-based one by more than about a week. This calculator shows the plain Naegele's-rule arithmetic; a clinical due date may be refined from there.
References
- ACOG Committee Opinion — Methods for Estimating the Due Date
- ACOG FAQ — When Pregnancy Goes Past Your Due Date
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.