When a doctor or midwife gives you a due date at your very first prenatal visit, they’re almost certainly using a formula that’s over 200 years old. Naegele’s rule is the classic method for estimating a due date from the first day of your last menstrual period, and despite modern ultrasound technology, it’s still the starting point nearly every pregnancy dating conversation begins with.
Where Naegele’s Rule Comes From
Naegele’s rule is named after Franz Karl Naegele, a German obstetrician who published the formula in 1806. It builds on the assumption — already established by earlier physicians — that pregnancy lasts about 280 days, or 40 weeks, from the first day of the last menstrual period (LMP), even though actual conception typically happens roughly two weeks after that date. In other words, “40 weeks pregnant” really means about 38 weeks since conception, not 40.
It’s worth noting that Franz Naegele himself didn’t originate the 280-day, 10-lunar-month idea from scratch — earlier German and Dutch physicians and midwives had used similar counting methods for generations. What Naegele did was formalize the calculation into a clean, teachable formula, which is a large part of why it spread so widely through medical training and stuck around for two centuries.
The Naegele’s Rule Formula
There are two mathematically identical ways to apply Naegele’s rule:
Method 1 — Add forward:
Take the first day of your LMP, add 9 months, then add 7 days.
Method 2 — Subtract and add:
Take the first day of your LMP, subtract 3 months, add 1 year, then add 7 days.
Both methods land on exactly the same date. The second version is the one most commonly taught in nursing and medical training because subtracting 3 months from any date is arithmetically simpler than adding 9.
A Worked Example
Say the first day of your last menstrual period was March 10.
- Method 1: March 10 + 9 months = December 10. December 10 + 7 days = December 17.
- Method 2: March 10 − 3 months = December 10. December 10 + 1 year = December 10 (next year). December 10 + 7 days = December 17.
Either way, the estimated due date is December 17 — exactly 280 days after March 10.
Why Naegele’s Rule Assumes a 28-Day Cycle
The formula only works cleanly because it assumes ovulation happens on day 14 of a standard 28-day menstrual cycle, with fertilization occurring shortly after and implantation following about a week later. That assumption is baked directly into the “add 280 days” math.
This is also Naegele’s rule’s biggest weakness. If your cycles are shorter than 28 days, ovulation happens earlier than day 14, so your real due date is likely to be earlier than what the formula predicts. If your cycles run longer than 28 days, ovulation happens later, and your real due date is likely to be later than the formula suggests. A person with consistently 35-day cycles, for example, ovulates roughly a week later than the 28-day assumption — which can shift the true due date by about that same week.
How Accurate Is Naegele’s Rule?
Naegele’s rule gives a reasonable estimate for people with regular 28-day cycles and a clearly remembered LMP date, but research reviewing the formula has found that fewer than 5% of pregnancies actually deliver on the exact predicted due date — most babies arrive within a window of roughly two weeks before to two weeks after it. That’s expected and normal; the “due date” was never meant to be a precise prediction, just a useful midpoint for planning.
For this reason, once a first-trimester ultrasound is performed, most clinicians will use the ultrasound-based due date instead of (or to confirm) the Naegele’s rule estimate, since measuring the embryo directly is generally considered more accurate, especially for people with irregular cycles or an uncertain LMP date — a preference laid out in ACOG’s committee opinion on methods for estimating the due date.
Naegele’s Rule vs. the Mittendorf-Williams Rule
A lesser-known alternative, the Mittendorf-Williams rule, was developed later using a larger, more modern dataset and suggests an average pregnancy length closer to 288 days for first-time mothers (and slightly less for subsequent pregnancies) rather than the flat 280 days Naegele’s rule uses. It hasn’t replaced Naegele’s rule in everyday clinical use, largely because Naegele’s simpler 280-day version remains the standard taught and charted, but it’s a useful reminder that “280 days” is itself an approximation, not a hard biological constant.
Frequently Asked Questions
Is Naegele’s rule still used by doctors today?
Yes. Naegele’s rule (or the equivalent 280-day calculation built into due-date calculators and electronic health records) remains the standard first-pass method for estimating a due date from LMP, even though ultrasound dating often refines or replaces it later in the first trimester.
How accurate is Naegele’s rule for irregular cycles?
Less accurate than for regular 28-day cycles. Because the formula assumes ovulation on day 14, anyone with cycles significantly shorter or longer than 28 days will get a due date estimate that’s systematically early or late by roughly the same number of days their cycle differs from 28.
Does Naegele’s rule account for the actual conception date?
No — it works entirely from the first day of the last menstrual period, not from a known or estimated conception date. If you know your conception date precisely (for example, from ovulation tracking or a single instance of intercourse), a conception-based due date estimate will generally be more accurate for you than Naegele’s rule.
What’s the difference between Naegele’s rule and an ultrasound due date?
Naegele’s rule calculates a due date purely from LMP and an assumed cycle length. An ultrasound due date is based on directly measuring the embryo or fetus, which sidesteps any uncertainty about cycle length or LMP recall entirely — that’s why it’s generally considered the more accurate method once it’s available.
Try the Modern, Adjustable Version
Doing this math by hand works, but see the modern, adjustable version of this calculation with our free pregnancy due date calculator, which applies the same 280-day logic while letting you adjust for your actual cycle length or switch to a conception-date or ultrasound-based estimate.
