OnlineCalculatorYou can calculate your due date by adding 280 days (40 weeks) to the first day of your last menstrual period. This method, known as Naegele's Rule, is the standard used by doctors worldwide and remains the most common starting point for estimating delivery. What most people don't know is that only about 5% of babies are born on their actual due date. The remaining 95% arrive within a two-week window before or after. That's why understanding all three calculation methods gives you a far more accurate picture of when to expect your baby. Use an online calculator to get your estimated date quickly.
What Is a Due Date?
A due date — medically called the Estimated Date of Delivery (EDD) — is the projected date your baby is expected to arrive. It marks the completion of 40 weeks of gestation, counted from the first day of your last menstrual period (LMP).
The World Health Organization (WHO) defines a full-term pregnancy as one lasting between 39 weeks and 40 weeks, 6 days. Babies born between 37 and 38 weeks, 6 days are classified as "early term." Those born after 41 weeks are "late term."
Your due date is an estimate, not a deadline. The American College of Obstetricians and Gynecologists (ACOG) recommends using a combination of LMP and ultrasound data for the most reliable EDD.
How to Calculate Your Due Date — 3 Methods
Each method uses different data points. Here's exactly how all three work.
Method 1: Naegele's Rule (LMP-Based)
This is the most widely used formula. Take the first day of your last menstrual period, add 7 days, then count forward 9 months (or simply add 280 days).
Example: If your LMP started on March 1, 2025, your due date would be December 6, 2025.
This method assumes a 28-day menstrual cycle and ovulation on day 14. If your cycle is longer or shorter, the estimate may shift.
Method 2: Ultrasound Dating
A first-trimester ultrasound (between weeks 6 and 13) measures the baby's crown-rump length (CRL). This measurement is accurate to within ± 5 to 7 days, making it the most precise method available.
ACOG guidelines state that if the ultrasound date differs from the LMP date by more than 7 days in the first trimester, the ultrasound date should replace the LMP-based EDD.
Method 3: IVF Transfer Date
For IVF (In Vitro Fertilization) pregnancies, the calculation is exact. Add 266 days to the egg retrieval or fertilization date. Alternatively, for a Day 5 blastocyst transfer, add 261 days to the transfer date.
This method removes cycle-length guesswork entirely.
Comparison Table: Due Date Calculation Methods
| Feature | Naegele's Rule (LMP) | Ultrasound Dating | IVF Transfer Date |
|---|---|---|---|
| Data Needed | First day of last period | Crown-rump length scan | Transfer or retrieval date |
| Accuracy | ± 14 days | ± 5–7 days (1st trimester) | ± 1–3 days |
| Best For | Regular 28-day cycles | Irregular cycles | IVF pregnancies |
| Cost | Free | $200–$500 (varies globally) | Included in IVF treatment |
| When Used | Any time | Weeks 6–13 ideal | From transfer date |
Expert insight: Many practitioners now use a "best EDD" approach, averaging the LMP and ultrasound dates when both fall within a 7-day range. This blended method is gaining traction in global obstetric practice.
Due Date Calculation — Key Facts and Figures
- A full-term pregnancy lasts 280 days or 40 weeks from LMP, per WHO and ACOG standards.
- Only 5% of babies arrive on the exact due date, according to data published by the National Institutes of Health (NIH).
- First-time mothers deliver an average of 1 to 2 days past their due date. Second-time mothers tend to deliver slightly earlier.
- ACOG recommends that ultrasound dating in the first trimester should take priority over LMP dating if there's a discrepancy greater than 7 days.
- The International Federation of Gynecology and Obstetrics (FIGO) considers any pregnancy lasting beyond 42 weeks (294 days) as "post-term," requiring increased monitoring.
⚠️ NOTE: Many women miscalculate their due date because they count from the date of conception rather than the first day of their last period. Gestational age starts from LMP, not from ovulation or intercourse. This two-week difference causes widespread confusion.
Key Factors to Consider
- Cycle Length Matters: Naegele's Rule assumes a 28-day cycle. If your cycle runs 35 days, your ovulation likely occurred around day 21, not day 14. This shifts your due date by about one week later. Adjust the formula accordingly or rely on ultrasound confirmation.
- Accuracy Decreases Over Time: Ultrasound dating is most accurate in the first trimester. By the third trimester, the margin of error grows to ± 21 days. Early scans give the best data.
- Multiple Pregnancies: Twins and multiples typically arrive earlier. The average delivery for twins is around 36 weeks. Your provider may set an earlier target date based on your specific situation.
- Medical Conditions: Conditions like gestational diabetes, preeclampsia, or placenta previa may lead to earlier planned delivery. Your due date becomes a flexible target rather than a fixed one.
- Irregular Periods: If your cycles are unpredictable or you were using hormonal contraception recently, LMP-based dating is unreliable. An early ultrasound becomes essential.
Common Mistakes People Make
- Counting from conception date instead of LMP. Gestational age begins two weeks before most women actually conceive. Using the conception date gives a due date that's two weeks too late. Always start from the first day of your last period.
- Trusting a late-pregnancy ultrasound for dating. A third-trimester ultrasound can be off by up to 3 weeks. Babies grow at different rates in later months. Only first-trimester scans are reliable for due date calculation.
- Assuming the due date is the delivery date. Only 5% of babies arrive on the exact EDD. ⚠️ WARNING: Making rigid plans around a single date — like scheduling work leave starting exactly on your due date — often leads to stress and logistical problems. Plan for a window of 37 to 42 weeks.
- Ignoring cycle length variations. A woman with a 35-day cycle ovulates about a week later than someone with a 28-day cycle. Using the standard formula without adjusting for this creates an EDD that's 7 days too early.
- Relying only on phone apps without medical confirmation. Pregnancy apps use basic algorithms. They don't account for irregular cycles, ultrasound data, or medical factors. Always confirm your EDD with a healthcare provider during your first prenatal visit.
Practical Tips
Use a trusted online calculator first. Before your first appointment, use a reliable online calculator to get a preliminary estimate. This helps you prepare questions for your doctor and understand roughly how far along you are.
Book an early ultrasound between weeks 7 and 9. This window offers the most accurate dating. Crown-rump length measurements at this stage are precise to within 5 days. Don't wait until the anatomy scan at week 20.
Record your cycle data before trying to conceive. Track at least 3 months of cycle lengths. Knowing your average cycle length makes Naegele's Rule far more accurate and helps your doctor assess your dates.
Ask your provider which EDD they're using. Sometimes the LMP date and ultrasound date differ. Confirm which one is in your medical file. This affects everything from screening test timing to labor induction scheduling.
Plan around a 5-week delivery window. Instead of fixating on one date, prepare for delivery anytime between weeks 37 and 42. Have your hospital bag packed by week 36. Arrange work and childcare coverage for this full range.
Adjust for longer cycles using the Mittendorf-Williams Rule. This lesser-known formula accounts for cycle length and whether you're a first-time mother. For first-time mothers, it estimates 288 days from LMP instead of 280. Discuss this with your provider if you have consistently long cycles.