What is Naegele’s rule?

Naegele’s rule is a traditional way to estimate a due date by adding 280 days (40 weeks) to the first day of the last menstrual period. It is a teaching tool and a common default in calculators—not a guarantee that ovulation happened on day 14 for every person. Historically it helped standardize classroom teaching and paper wheels; digitally, the same arithmetic appears as “LMP mode” in many apps.

The rule in one sentence

LMP day one + 280 days ≈ estimated due date. Many textbooks present it as a pregnancy wheel shortcut. Our LMP calculator page applies the same family of thinking in digital form.

Hidden assumptions (why it can be “off”)

The rule assumes a 28-day cycle and ovulation around mid-cycle. If your ovulation is earlier or later, the true conception date does not line up with the LMP story the same way—yet the rule still prints a neat calendar day.

Why teaching rules persist in software

Engineers need a deterministic default. Naegele-style math is easy to code, easy to explain in help text, and matches what many patients saw in pamphlets. That does not mean your body read the same textbook—if your cycles vary, pair the rule with ultrasound dating guidance from your team.

Connect to how due dates are calculated and what conception date means.

How clinicians use the rule today

Even where Naegele-style math is taught, real practice often prioritizes ultrasound when it is more reliable. Think of the rule as a baseline, not a final court verdict.

Why it still matters for learning

Because so many charts and pamphlets reference it, understanding Naegele helps you decode what a quick app is doing under the hood. That literacy makes conversations with your team faster.

Examples and quick calculations

Quick check: If LMP is April 4, adding 280 days lands around January 9 next year (verify with a tool—leap years and timezone matter at margins).

Worked idea: If someone says “add nine months,” calendar months are uneven; adding 280 days is more reproducible across February lengths and 30- vs 31-day months.

Common misconceptions

• Mistake: calling it “exact science.” It is a population-level shortcut.
• Mistake: counting from the last day of bleeding. LMP means first day of the period before pregnancy.
• Mistake: assuming Naegele replaces IVF math. See the IVF FAQ for transfer-based rules.

Planning tips (non-medical)

• If your cycles are often 35+ days, mention it early—your team may lean on ultrasound sooner.
• Read our blog guide for a slower walkthrough: how to calculate your pregnancy due date (also linked from the FAQ hub).

Related guides and tools

Use these internal links to keep learning—each FAQ is written to stand alone, and the calculators help you turn reading into concrete numbers for your next appointment.

Pregnancy due date calculator (home) · Due date by LMP · Pregnancy week calculator · How many weeks pregnant · Blog: calculate due date · Blog index · About

Last reviewed for clarity: May 2026. Always follow your own clinician’s dating, screening schedule, and urgent-care instructions.