What is Naegele's rule?

Naegele's rule is the standard obstetric formula for estimating a due date: take the first day of the last menstrual period, subtract 3 months, then add 7 days (equivalent to adding 280 days). It assumes a 28-day cycle with ovulation on day 14. This calculator automatically adjusts for cycles that differ from 28 days.

How accurate is the LMP-based due date?

LMP-based dating is accurate to within 1–2 weeks for most women with regular cycles. Only about 5% of babies are born on their exact due date. An early ultrasound (before 14 weeks) typically provides more accurate dating and may revise the LMP estimate.

Why does cycle length matter?

Standard Naegele's rule assumes a 28-day cycle. If your cycle is shorter or longer, ovulation (and therefore conception) shifts accordingly. This calculator adds or subtracts the difference from 28 days to the standard 280-day count, giving a corrected EDD.

When does each trimester begin?

The first trimester spans weeks 1 through 12. The second trimester covers weeks 13 through 26. The third trimester runs from week 27 until delivery. These boundaries are consistent across standard obstetric practice.

What is the difference between gestational age and fetal age?

Gestational age counts from the first day of the LMP — typically about two weeks before conception. Fetal age (embryonic age) counts from actual fertilization. Healthcare providers almost always use gestational age because the LMP date is easier to establish than the exact conception date.

Can I use this calculator after an ultrasound has given a different due date?

If your ultrasound dating differs from the LMP estimate by more than 5–7 days, your provider will typically adjust the official EDD to match the ultrasound. In that case, use the ultrasound-based date as your reference rather than the LMP calculation.

Pregnancy Due Date (LMP) Calculator

Name: Pregnancy Due Date (LMP) Calculator
Availability: InStock
Author: Nham Vu

Enter the first day of your last menstrual period to calculate your estimated due date, current gestational age, trimester, and key pregnancy milestones using Naegele's rule.

LMP Information

First Day of Last Menstrual Period

Enter the first day of your most recent period.

Average Cycle Length (days)

Standard is 28 days. Valid range: 21–45 days.

Naegele's Rule: EDD = LMP + 280 days (adjusted for cycle length). Ovulation is assumed at day (cycle ÷ 2) of your cycle.

Enter your LMP date and click Calculate

Your estimated due date will appear here

Please enter a valid date.

Summary

Enter the first day of your last menstrual period to calculate your estimated due date, current gestational age, trimester, and key pregnancy milestones using Naegele's rule.

How it works

Enter the first day of your last menstrual period (LMP) in the date field.
Adjust the average cycle length if it differs from the standard 28 days (range: 21–45 days).
Click "Calculate" to instantly see your estimated due date using Naegele's rule.
Review your current gestational age in weeks and days, and your trimester.
Scroll down to see upcoming pregnancy milestones with their expected dates.

Use cases

Calculate your estimated due date before your first prenatal visit.
Confirm gestational age based on your last menstrual period.
Determine which trimester you are currently in.
Plan around key prenatal appointments and screening windows.
Verify or cross-check due dates discussed with your healthcare provider.
Understand how a non-standard cycle length shifts your EDD.

Frequently Asked Questions

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Last updated: 2026-05-23 · Reviewed by Nham Vu