Due Date Calculator

Naegele's Rule • 2026 Edition

Naegele's Rule Formula:

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Due Date = LMP + 7 days - 3 months + 1 year

Where:

  • LMP: Last Menstrual Period (first day of last period)
  • + 7 days: Accounts for average ovulation timing
  • - 3 months: Gestational period adjustment
  • + 1 year: Full pregnancy cycle

Alternative Formula: LMP + 280 days (40 weeks)

Adjustment for Irregular Cycles: Due Date + (Cycle Length - 28 days)

This assumes a 28-day cycle with ovulation on day 14. For greater accuracy, use ultrasound measurements in early pregnancy.

Pregnancy Dates

Standard: 28 days (adjust if different)

Ultrasound Data

Advanced Options

Due Date Results

Oct 22, 2025
Estimated Due Date
8
Current Week of Pregnancy
300
Days Until Due
Jan 29, 2024
Estimated Conception
Week 8 of 40 (20%)
Primary Method: Naegele's Rule

Based on LMP: January 15, 2024

Formula: LMP + 7 days - 3 months + 1 year

Trimester Schedule

1st Trimester: Jan 15 - May 8, 2024

2nd Trimester: May 9 - Sep 29, 2024

3rd Trimester: Sep 30, 2024 - Oct 22, 2025

Method Due Date Confidence Notes
Naegele's Rule Oct 22, 2025 Medium Based on LMP
Ultrasound Adjusted Oct 22, 2025 High If available
Conception Based Oct 22, 2025 High If known
Naegele's Rule

Traditional method: LMP + 7 days - 3 months + 1 year

Assumes 28-day cycle with ovulation on day 14

Conception Method

Conception date + 266 days (38 weeks)

More accurate when conception date is known

Ultrasound Method

Based on fetal measurements

Most accurate in first trimester

Accuracy Guidelines
  • First trimester ultrasound: ±5-7 days accuracy
  • Naegele's rule: ±14 days accuracy
  • Conception date: ±2-3 days accuracy

Due Date Calculation Guide

Naegele's Rule Explained

Naegele's Rule, developed by Dr. Franz Carl Naegele in the 1800s, is the standard method for calculating estimated due dates in obstetrics. It assumes a regular 28-day menstrual cycle with ovulation occurring on day 14.

Due Date Calculation Methods

Several methods can be used to calculate due dates:

Naegele's Rule: LMP + 7 days - 3 months + 1 year
Alternative: LMP + 280 days (40 weeks)
Conception Method: Conception Date + 266 days (38 weeks)

Where:

  • LMP: First day of Last Menstrual Period
  • 280 days: Standard gestational period (40 weeks)
  • 266 days: Fetal development period (38 weeks)

Calculation Accuracy
1
First Trimester Ultrasound: Most accurate method (±5-7 days)
2
Known Conception Date: Highly accurate (±2-3 days)
3
Naegele's Rule: Standard method (±14 days)
Trimester Breakdown

Due dates help organize pregnancy into three distinct periods:

  • First Trimester: Weeks 1-12 (organ development)
  • Second Trimester: Weeks 13-27 (growth phase)
  • Third Trimester: Weeks 28-40 (maturation phase)
Due Date Tips
  • Confirm early: First-trimester ultrasound provides best accuracy
  • Track cycles: Know your average length for adjustments
  • Consider irregularities: Adjust for cycles shorter/longer than 28 days
  • Plan appointments: Schedule around key milestones
  • Prepare mentally: Use due date as planning tool, not deadline

Due Date Basics

Estimated Due Date (EDD)

The calculated date when delivery is expected, based on LMP and cycle length.

Naegele's Rule

LMP + 7 days - 3 months + 1 year

Standard 280-day (40-week) gestation period.

Key Rules:
  • Assumes 28-day cycle with ovulation on day 14
  • Actual delivery within 2 weeks is normal
  • Ultrasound provides more accurate dating

Calculation Methods

Multiple Approaches

Different methods for different situations and accuracy levels.

Method Selection
  1. Naegele's Rule (standard)
  2. Ultrasound measurements
  3. Conception date
  4. IVF transfer date
Accuracy Considerations:
  • Earlier ultrasounds are more accurate
  • Irregular cycles require adjustments
  • Conception date is most precise
  • Due date is an estimate, not guarantee

Due Date Calculation Quiz

Question 1: Multiple Choice - Understanding Naegele's Rule

According to Naegele's Rule, what is the correct mathematical sequence for calculating an estimated due date?

Solution:

The answer is B) LMP + 7 days - 3 months + 1 year. This is the standard Naegele's Rule formula used in obstetrics. The sequence adds 7 days to account for ovulation timing, subtracts 3 months to adjust for the gestational period, and adds 1 year to complete the full pregnancy cycle.

Pedagogical Explanation:

Naegele's Rule simplifies the complex process of pregnancy dating by using a standardized formula. The addition of 7 days compensates for the typical 14-day ovulation cycle, while subtracting 3 months aligns with the approximate gestational period. The formula assumes a regular 28-day menstrual cycle with ovulation on day 14.

Key Definitions:

Naegele's Rule: Standard method for calculating estimated due dates

LMP: Last Menstrual Period - first day of last menstrual bleeding

EDD: Estimated Due Date - calculated delivery date

Important Rules:

• Sequence matters: add 7, subtract 3 months, add 1 year

• Assumes regular 28-day cycles

• Actual delivery within 2 weeks is considered normal

Tips & Tricks:

• Remember: Add 7, subtract 3, add 1

• Always use first day of LMP

• Confirm with early ultrasound

Common Mistakes:

• Reversing the order of operations

• Using the last day of period instead of first

• Forgetting to adjust for irregular cycles

Question 2: Due Date Formula Application

Calculate the estimated due date for a woman whose LMP was March 15, 2024. Show your work using Naegele's Rule.

Solution:

Using Naegele's Rule: LMP + 7 days - 3 months + 1 year

Given: LMP = March 15, 2024

Step 1: Add 7 days → March 15 + 7 = March 22, 2024

Step 2: Subtract 3 months → March 22 - 3 months = December 22, 2024

Step 3: Add 1 year → December 22, 2024 + 1 year = December 22, 2025

Therefore, the estimated due date is December 22, 2025.

Pedagogical Explanation:

This calculation demonstrates how Naegele's Rule accounts for the typical 280-day (40-week) gestational period. The formula works by adjusting the calendar to accommodate the biological timeline of pregnancy, assuming ovulation occurred 14 days after the LMP and fertilization followed shortly after.

Key Definitions:

Gestational Age: Time elapsed since first day of LMP

Conception Age: Time elapsed since fertilization

Full Term: Pregnancy lasting 37-42 weeks gestational age

Important Rules:

• Always use the first day of LMP

• Account for month boundaries (March to December)

• Add full year to reach due date year

Tips & Tricks:

• Break down into steps: add, subtract, add

• Use calendar to visualize month transitions

• Verify with online calculators

Common Mistakes:

• Adding months instead of subtracting

• Forgetting to add the year

• Miscounting days in months

Question 3: Word Problem - Irregular Cycle Adjustment

Sarah has an irregular menstrual cycle that averages 32 days instead of the standard 28 days. Her LMP was February 10, 2024. Calculate her adjusted due date considering her longer cycle. (Hint: Standard gestation is 280 days, so adjust for the 4 extra days per cycle)

Solution:

Step 1: Calculate standard due date using Naegele's Rule

February 10, 2024 + 7 days = February 17, 2024

February 17, 2024 - 3 months = November 17, 2024

November 17, 2024 + 1 year = November 17, 2025

Step 2: Adjust for longer cycle (32 vs 28 days)

Difference per cycle = 32 - 28 = 4 days longer

Since her cycle is longer, ovulation occurs later, so due date should be later

Adjusted due date = November 17, 2025 + 4 days = November 21, 2025

Therefore, Sarah's adjusted due date is November 21, 2025.

Pedagogical Explanation:

Women with longer cycles typically ovulate later in their cycle, which shifts the conception date and subsequently the due date. The adjustment accounts for the delayed ovulation. For every day the cycle is longer than 28 days, the due date moves forward by one day.

Key Definitions:

Irregular Cycle: Menstrual cycle varying more than 7-9 days

Ovulation Timing: Day of egg release, varies with cycle length

Conception Window: Days when pregnancy is possible

Important Rules:

• Longer cycles = later ovulation = later due date

• Shorter cycles = earlier ovulation = earlier due date

• Adjust by the difference from 28 days

Tips & Tricks:

• Track cycles for 3-6 months to find average

• Use ovulation predictor kits for accuracy

• Confirm with early ultrasound

Common Mistakes:

• Not adjusting for irregular cycles

• Adjusting in wrong direction

• Using single cycle length instead of average

Question 4: Application-Based Problem - Ultrasound Adjustment

A pregnant woman's LMP was January 15, 2024, giving a due date of October 22, 2025 using Naegele's Rule. However, an ultrasound at 12 weeks shows the fetus is measuring at 13 weeks and 2 days. How should the due date be adjusted based on the ultrasound measurement?

Solution:

Step 1: Calculate the discrepancy

Ultrasound shows 13 weeks 2 days, but LMP suggests 12 weeks

Discrepancy = 1 week 2 days (9 days)

Step 2: Determine the adjustment

The ultrasound indicates the pregnancy is more advanced than LMP suggests

Adjust due date earlier by the discrepancy amount

Step 3: Calculate new due date

Original due date: October 22, 2025

New due date: October 22, 2025 - 9 days = October 13, 2025

Therefore, the ultrasound-adjusted due date is October 13, 2025.

Pedagogical Explanation:

Ultrasound measurements in the first trimester are more accurate than LMP-based calculations. When there's a significant discrepancy (usually >7 days in first trimester), healthcare providers typically adjust the due date based on ultrasound measurements. This reflects the actual developmental age of the fetus.

Key Definitions:

Ultrasound Dating: Using fetal measurements to estimate gestational age

Measurement Discrepancy: Difference between LMP and ultrasound dates

Adjustment Criteria: Threshold for changing due date

Important Rules:

• First trimester ultrasounds are most accurate

• Adjust if discrepancy >7 days in first trimester

• Later ultrasounds are less reliable for dating

Tips & Tricks:

• First trimester ultrasounds take precedence

• Record all dates provided by healthcare

• Understand dating may change

Common Mistakes:

• Ignoring ultrasound adjustments

• Not understanding accuracy differences

• Resisting due date changes

Question 5: Multiple Choice - Due Date Accuracy

Which method provides the most accurate estimated due date in clinical practice?

Solution:

The answer is B) First-trimester ultrasound. Ultrasound measurements in the first trimester provide the most accurate due date estimates, with a margin of error of ±5-7 days, compared to ±14 days for Naegele's Rule. Fetal measurements correlate directly with developmental age.

Pedagogical Explanation:

First-trimester ultrasounds are considered the gold standard for due date determination because fetal measurements at this stage correlate very closely with actual gestational age. As pregnancy progresses, variations in fetal growth make dating less precise. The crown-rump length measured in early ultrasound is the most reliable indicator of gestational age.

Key Definitions:

Accuracy Margin: Range of possible error in dating methods

Gold Standard: Most reliable method available

Crown-Rump Length: Measurement from top of head to bottom of spine

Important Rules:

• First trimester ultrasound: ±5-7 days accuracy

• Naegele's rule: ±14 days accuracy

• Later ultrasounds less reliable for dating

Tips & Tricks:

• Schedule early ultrasound for accurate dating

• Record all provided dates

• Understand due dates may change

Common Mistakes:

• Relying solely on LMP for dating

• Not getting early ultrasound

• Resisting date changes when indicated

Due Date Calculator

FAQ

Q: How accurate is the due date calculated by Naegele's Rule?

A: Naegele's Rule has a margin of error of approximately ±14 days (about 2 weeks). The rule assumes:

  • A 28-day cycle with ovulation on day 14
  • 40-week (280-day) gestation period
  • Regular, predictable ovulation timing

Only about 4% of women deliver on their exact due date. For more accuracy, early ultrasounds (especially before 12 weeks) provide better dating by measuring fetal crown-rump length with a margin of error of ±5-7 days.

Q: When should due dates be changed based on ultrasound measurements?

A: Clinical guidelines recommend adjusting due dates when there's a significant discrepancy between LMP-based dates and ultrasound measurements:

  • First trimester: Change if discrepancy >7 days
  • Second trimester: Change if discrepancy >10 days
  • Third trimester: Change if discrepancy >21 days

First-trimester ultrasounds are most reliable for dating because fetal measurements correlate closely with actual gestational age during this period.

About

Obstetric Standards
This calculator follows Naegele's Rule and standard obstetric practices. Calculations are based on evidence-based guidelines. Results should be confirmed with healthcare providers. Updated: Jan 2026.