Naegele’s Rule: The Formula Behind Your Due Date

What Is Naegele’s Rule?

Naegele's Rule is a method for estimating the due date of a pregnancy: take the first day of the last menstrual period, add one year, subtract three months, and add seven days. The result is the Estimated Due Date (EDD) — equivalent to 280 days (40 weeks) after the LMP.

What is remarkable is not the complexity of the rule — a child could do the arithmetic — but its longevity. Formulated before ultrasound, before hormones were understood, and before anyone knew what ovulation was, it still sits at the heart of modern obstetrics, now confirmed and refined by first-trimester scans.

The History: Franz Naegele and 1812

Franz Karl Naegele (1778–1851) was a professor of obstetrics at the University of Heidelberg in Germany. Drawing on clinical observation, he noted that human pregnancies lasted roughly ten lunar months from the last menstrual period — ten cycles of 28 days, or 280 days — and formalized the pattern into the rule that bears his name.

He published it in his 1812 obstetrics textbook, and it spread rapidly through European medicine — partly because it was simple enough to use at the bedside with no instruments at all. Interestingly, some historians note that the observation predates Naegele; similar “ten lunar months” reckonings appear in earlier medical writing. Naegele’s contribution was systematizing it into the clean year-minus-months-plus-days formula that clinicians could apply uniformly.

Two centuries later, the rule has survived every technological revolution in obstetrics. Modern population data puts average gestation from the LMP very close to Naegele’s 280 days — validating an observation made with nothing but careful record-keeping. Ultrasound has not replaced the rule; it has become its partner, confirming or correcting the LMP-based estimate in early pregnancy.

The Formula, Step by Step

The classic form of the rule is a small piece of calendar arithmetic:

Why does this particular dance work? Adding a year and subtracting three months moves you forward about nine calendar months — roughly 273 days. The final +7 days tops it up to 280 days, the observed average gestation. It is just a memorizable shortcut for “LMP + 280 days” that people could do in their heads in 1812.

In modern practice, most clinicians and calculators skip the year-months-days choreography and add 280 days directly — or use the conception-based equivalent, conception date + 266 days, since conception typically occurs about 14 days after the LMP. All three forms are the same rule wearing different clothes.

Worked Examples (Math Checked)

Example 1: Standard 28-day cycle

LMP = January 1, 2026

1. Add 1 year → January 1, 2027
2. Subtract 3 months → October 1, 2026
3. Add 7 days → October 8, 2026

EDD = Thursday, October 8, 2026. Check: January 1, 2026 plus 280 days is also October 8, 2026 — the two forms agree exactly.

Example 2: Longer cycle (35 days)

The rule assumes a 28-day cycle, so a 35-day cycle needs a +7 day correction (35 − 28 = 7):

1. LMP = January 1, 2026; base EDD = October 8, 2026
2. Add the cycle difference: 7 days → October 15, 2026

EDD = Thursday, October 15, 2026. Equivalently: January 1, 2026 + 287 days = October 15, 2026.

Example 3: Shorter cycle (21 days)

A 21-day cycle needs a −7 day correction (21 − 28 = −7):

1. LMP = January 1, 2026; base EDD = October 8, 2026
2. Subtract the cycle difference: 7 days → October 1, 2026

EDD = Thursday, October 1, 2026 (January 1, 2026 + 273 days).

Apply Naegele’s Rule with our calculator →

The Cycle-Length Adjustment

Naegele’s original rule silently assumes ovulation on day 14 of a 28-day cycle — but the biologically important interval is the luteal phase, the stretch between ovulation and the next period, which is fairly constant at about 14 days. That means ovulation actually occurs around day (cycle length − 14): day 21 in a 35-day cycle, day 7 in a 21-day cycle.

The standard correction captures this: EDD = LMP + 280 + (cycle length − 28) days. Longer cycle → later ovulation → later due date; shorter cycle → earlier ovulation → earlier due date. It is an approximation, not a measurement — real luteal phases vary from roughly 11 to 17 days — but it corrects the biggest systematic error in the rule.

If you know your actual ovulation day from tracking (LH tests, basal body temperature, cervical mucus charting), skip the adjustment entirely and treat ovulation day as your conception date: add 266 days. A measured ovulation beats an assumed one every time.

The Limitations of Naegele’s Rule

Every assumption in the rule is a potential source of error:

• The 28-day cycle assumption. Many women have consistently longer or shorter cycles, and the unadjusted rule will systematically misdate them by the difference.
• Day-14 ovulation. Even in regular cycles, ovulation day varies month to month and person to person; the luteal phase is not exactly 14 days for everyone.
• LMP recall. The rule is only as good as the date you feed it. Light bleeding can be mistaken for a period, and many people simply misremember by a few days.
• Biological variation. Healthy gestations naturally span roughly 37–42 weeks. The rule estimates the average, so about half of pregnancies would “disagree” with it even with perfect inputs.
• Parity effects. Research suggests first pregnancies average slightly longer than the rule predicts — the basis for the Mittendorf-Williams update described below.

None of this makes the rule useless — it makes it a first estimate. Modern practice treats Naegele’s Rule as the opening bid, confirmed or corrected by first-trimester ultrasound, which is why the combination remains the global standard.

When Naegele’s Rule Gets It Wrong

There are situations where the rule should not be trusted at all, and knowing them can save you weeks of confusion:

• Irregular cycles or PCOS — when ovulation timing is unpredictable, the day-14 assumption collapses. An early dating scan is the answer.
• Recent hormonal contraception — coming off the pill or an IUD can mean anovulatory or unusually timed first cycles.
• Conceiving while breastfeeding — ovulation can return before the first postpartum period, leaving no LMP to anchor the math.
• Uncertain LMP — if you are guessing the date within a week or two, the rule inherits all of that uncertainty.
• IVF pregnancies — the embryo’s exact age is known, so transfer-date math (±1–3 days) beats Naegele’s assumptions outright.

In all of these cases, the fix is the same: a first-trimester ultrasound. A crown-rump length measurement between 8 and 14 weeks dates the pregnancy to about ±5 days regardless of cycle history — no assumptions required. If your situation matches any item on this list, mention it at your first prenatal visit so dating is handled by scan from the start.

What Came After: Updates and Alternatives

The most notable challenger is the Mittendorf-Williams Rule (1990), derived from a large study of pregnancy lengths. It keeps Naegele’s skeleton but adds parity-based corrections: about +15 days for first-time mothers and +10 days for those who have given birth before, on top of cycle-length adjustments. Research suggested it predicted actual delivery dates slightly better — but it never displaced Naegele’s Rule in routine clinical practice, largely because the improvement was modest and the original was already embedded everywhere.

The more consequential “update” is not a formula at all: ultrasound dating. Guidelines now recommend confirming every Naegele’s-based date with a first-trimester scan, and replacing it when the two differ by more than 7 days (11–14 weeks). In effect, modern dating is a hybrid — Naegele’s Rule proposes, ultrasound disposes.

For a hands-on walkthrough of all the dating methods side by side, see our step-by-step guide to calculating your due date, or explore each week of the journey in the pregnancy week-by-week guide.