How to Use the Pregnancy Due-Date (Naegele) Calculator
The Pregnancy Due-Date (Naegele) calculator is a digital tool that applies the standard obstetric formula to estimate when your baby will be born. Named after German obstetrician Hermann Ludwig Ferdinard von Haus Naegele, this method has been used since the 1800s to predict delivery dates based on the first day of your last menstrual period. Understanding your estimated due date helps you prepare for pregnancy milestones, plan maternity leave, and set realistic expectations for when labor may begin.
To use the calculator, you will need to enter the first day of your last menstrual period (LMP) as the primary input. The calculator assumes a standard 28-day menstrual cycle and ovulation on day 14, which applies to approximately 60-70% of women with regular cycles. If your cycle is typically shorter than 24 days or longer than 32 days, notify your healthcare provider, as the standard Naegele calculation may be less accurate for your situation.
The calculator will display your estimated due date along with the corresponding week and day of pregnancy. This date represents the 280-day (40-week) mark from your LMP, though only about 4% of pregnancies actually deliver on this exact date. The result should be used as a target rather than a guarantee; most deliveries occur within a 2-week window (±14 days) around the calculated due date. Always confirm your due date with your healthcare provider through clinical examination and ultrasound, especially if your LMP date is uncertain or your cycle is irregular.
Naegele's Rule Calculation Examples
This table demonstrates how to apply Naegele's Rule to various last menstrual period dates to arrive at estimated due dates.
| Last Menstrual Period (LMP) | Month +3 / Year +1 | Add 7 Days | Estimated Due Date |
|---|---|---|---|
| January 1, 2024 | April 1, 2024 | April 8, 2024 | October 8, 2024 |
| March 15, 2024 | June 15, 2024 | June 22, 2024 | December 22, 2024 |
| June 20, 2024 | September 20, 2024 | September 27, 2024 | March 27, 2025 |
| August 10, 2024 | November 10, 2024 | November 17, 2024 | May 17, 2025 |
| November 5, 2024 | February 5, 2025 | February 12, 2025 | August 12, 2025 |
This method assumes a regular 28-day menstrual cycle. Adjust dates accordingly if your cycle length differs significantly.
Pregnancy Dating Accuracy by Timing of Ultrasound
Ultrasound dating accuracy varies significantly depending on the gestational age at which the scan is performed.
| Ultrasound Timing | Gestational Age Range | Accuracy Margin | Accuracy Rate |
|---|---|---|---|
| First Trimester | 8-13 weeks | <3-5 days | 95-98% |
| Early Second Trimester | 14-20 weeks | ±1-2 weeks | 90-95% |
| Late Second Trimester | 21-24 weeks | ±2-3 weeks | 85-90% |
| Third Trimester | 25-40 weeks | ±3-4 weeks | 75-85% |
First-trimester ultrasound is considered the gold standard for pregnancy dating when the LMP is uncertain or unreliable.
Menstrual Cycle Length Impact on Naegele's Rule
Naegele's Rule assumes a 28-day cycle, but actual due dates may shift based on individual cycle length variations.
| Cycle Length (Days) | Ovulation Day (Approximate) | Days Adjustment from Standard | Due Date Shift |
|---|---|---|---|
| 21 days | Day 7 | -7 days | Due date ~7 days earlier |
| 24 days | Day 10 | -4 days | Due date ~4 days earlier |
| 28 days (standard) | Day 14 | 0 days | No adjustment needed |
| 32 days | Day 18 | +4 days | Due date ~4 days later |
| 35+ days | Day 21+ | +7 days or more | Due date ~7+ days later |
Women with irregular cycles should rely on ultrasound dating rather than LMP-based calculation for accuracy.
Pro Tips
- Record the exact first day of your last menstrual period as soon as you suspect pregnancy — this single piece of information is the foundation of all Naegele Rule calculations and should be verified with your healthcare provider during your first prenatal visit.
- If your menstrual cycles are typically longer than 35 days or shorter than 21 days, ask your healthcare provider to adjust the Naegele calculation or rely on first-trimester ultrasound dating instead, as standard Naegele's Rule may be inaccurate by 1-3 weeks.
- Use your Naegele due date as a flexible target window rather than a fixed deadline — plan maternity leave for 2-4 weeks around the due date rather than on the exact date, as only 4% of births occur on the predicted day.
- Confirm your due date through a first-trimester ultrasound (ideally between weeks 8-13) when possible, as this method is accurate to within <3-5 days and more reliable than LMP-based calculation alone, especially if you have irregular cycles.
Common Mistakes to Avoid
Assuming the due date is a hard deadline
Many pregnant women believe they will definitely deliver on their due date, but in reality only about 4% of births occur on the exact predicted day. Pregnancies that extend to 41-42 weeks are considered normal, and your provider may not recommend induction unless medically necessary.
Forgetting to account for cycle length variations
Naegele's Rule assumes a 28-day cycle, but if your cycles are consistently longer or shorter, your actual due date could be off by 1-3 weeks. Women with cycles <21 days or >35 days should request adjusted dating from their healthcare provider.
Relying solely on LMP when the date is uncertain
If you cannot remember your exact LMP or have irregular periods, using Naegele's Rule will produce an unreliable estimate. First-trimester ultrasound is far more accurate and should be your primary dating method if LMP is unknown.
Ignoring ultrasound dating when it differs from Naegele's prediction
If your ultrasound dating differs from your LMP-based due date by more than 1 week, your healthcare provider should adjust your official due date to match the ultrasound findings, which are more accurate. Do not insist on the Naegele date if clinical imaging suggests otherwise.
Frequently Asked Questions
What is Naegele's Rule and how does it calculate my due date?
Naegele's Rule is the standard obstetric method for estimating pregnancy due date by adding 280 days (40 weeks) to the first day of your last menstrual period (LMP). This calculation assumes a typical 28-day menstrual cycle and ovulation occurring on day 14. The formula subtracts 3 months from your LMP and adds 1 year and 7 days to arrive at the estimated due date. Studies show approximately 4% of pregnancies deliver on the exact predicted date, with most deliveries occurring within 2 weeks of the estimate.
How accurate is the Naegele due date calculator?
Naegele's Rule is accurate within a 2-week window for approximately 68% of pregnancies when the LMP is known with certainty. Accuracy decreases to 50-60% for women with irregular menstrual cycles longer than 35 days or shorter than 21 days. Ultrasound dating performed in the first trimester is considered the gold standard with <3-5 day margin of error. For cycles significantly different from 28 days, your healthcare provider may adjust the due date accordingly.
Why do doctors use a 280-day pregnancy instead of 9 months?
Pregnancy is calculated as 280 days or 40 weeks from the first day of the last menstrual period, which equals approximately 9.5 calendar months rather than exactly 9 months. This standardized measurement accounts for the fact that ovulation (when conception actually occurs) typically happens around day 14 of the cycle, making the true gestational age about 2 weeks less than the time elapsed since LMP. Using the 280-day standard ensures consistency across medical providers and allows for better comparison of fetal development across populations.
What if I have an irregular menstrual cycle—does Naegele's Rule still work?
Naegele's Rule is less reliable for women with irregular cycles, as it assumes a consistent 28-day cycle with ovulation on day 14. Women with cycle lengths <21 days or >35 days may have inaccurate due dates using this method, with potential errors of 1-3 weeks. If you have irregular periods, an early ultrasound (typically between weeks 8-13) is recommended to confirm gestational age more accurately. Your healthcare provider may also ask about the date of conception if known or use other clinical markers to refine the estimate.
Can I calculate my due date if I don't remember my last menstrual period?
If you cannot recall your exact LMP, your healthcare provider will typically use first-trimester ultrasound dating, which is accurate to within <3-5 days when performed between weeks 8-13 of pregnancy. Second-trimester ultrasound (weeks 14-20) has an accuracy margin of ±1-2 weeks, while third-trimester ultrasound accuracy decreases to ±3-4 weeks. Keeping a menstrual calendar or using period-tracking apps can help you remember this important date for future pregnancies or medical consultations.
How does Naegele's Rule apply if I used fertility treatments or assisted reproduction?
For assisted reproductive technologies (ART) such as IVF, the due date is calculated from the egg retrieval or embryo transfer date rather than LMP, as the exact conception date is known. If the egg retrieval occurred on a specific date, add 266 days (38 weeks) to determine the due date, or add 280 days if calculating from the LMP before treatment. Fertility clinics typically provide a more precise due date than Naegele's Rule alone, so follow your reproductive endocrinologist's dating when available.
What is the difference between due date and estimated delivery date?
The due date is a single target date calculated 280 days from the LMP, while the estimated delivery date is often presented as a range spanning 2-4 weeks. Most healthcare providers now emphasize that only about 4% of births occur on the exact due date, with approximately 80% of deliveries happening between 38 and 42 weeks of gestation. Your provider may use terms like 'due window' or 'expected delivery range' to better set realistic expectations around when labor may begin.
Should I plan my maternity leave based on my Naegele due date?
Your Naegele due date provides a target for planning, but you should plan maternity leave with flexibility for a 2-4 week window on either side of the due date. Most women begin leave between 36-38 weeks of pregnancy, allowing time before the due date to prepare and rest. It's advisable to discuss timeline flexibility with your employer, as only about 5% of pregnancies deliver on the exact due date, and some pregnancies are induced or delivered early for medical reasons.
How does maternal age affect the accuracy of Naegele's Rule?
Naegele's Rule accuracy is not directly affected by maternal age, but women >35 years old may have different cycle patterns and higher rates of pregnancy complications that could influence delivery timing. Advanced maternal age (≥35 years) has slightly higher rates of spontaneous preterm birth and induction before 40 weeks, which may shift actual delivery dates earlier than the Naegele estimate. Regardless of age, ultrasound confirmation in the first trimester remains the most accurate way to establish due date across all maternal age groups.
References & Resources
Last updated: April 2026
- Pregnancy Dating – American College of Obstetricians and Gynecologists (ACOG)
Official ACOG guidance on accurate pregnancy dating methods including Naegele's Rule, ultrasound, and clinical examination.
- Naegele's Rule and Pregnancy Estimation – CDC Reproductive Health
CDC resource explaining pregnancy dating standards and the role of Naegele's Rule in obstetric practice.
- Ultrasound Accuracy for Dating Pregnancy – National Institutes of Health
NIH-indexed research on the accuracy and reliability of ultrasound compared to menstrual history for pregnancy dating.
- Understanding Your Due Date – WebMD Pregnancy Center
Patient-friendly information on how due dates are calculated and what to expect around your estimated delivery date.