Due Date Calculator
Calculate your estimated due date using Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period. Only about 5% of babies arrive on their exact due date.
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Key values and context for this tool: • Full Term: 40 Weeks • On Due Date: ~5% • Term Window: 37-42 Wks
Ready to run the numbers?
Why: Calculate your estimated due date using LMP, conception date, ultrasound, or IVF transfer.
How: Enter your values in the calculator fields below, keep units consistent, then run the calculation to see results and any step-by-step work shown on this page.
Run the calculator when you are ready.
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For informational purposes only — not medical advice. Consult a healthcare professional before acting on results.
A due date calculator estimates your Expected Date of Delivery (EDD) using Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period. Only about 5% of babies arrive on their exact due date; most are born within two weeks before or after. First-trimester ultrasound provides the most accurate dating (±5 days).
Sources: ACOG, WHO, NIH, CDC.
Key Takeaways
- • Naegele's rule: EDD = LMP + 280 days (+ cycle adjustment)
- • First-trimester ultrasound is more accurate than LMP for irregular cycles
- • IVF due dates use transfer date + 261 (Day 5) or 263 (Day 3) days
- • Full term is 39-40 weeks; early term 37-38; late term 41; post-term 42+
Did You Know?
How Does Due Date Calculation Work?
LMP Method
Add 280 days to the first day of your last period. For cycles ≠28 days, add (cycle length - 28).
Conception Method
Add 266 days to the known conception date. Pregnancy from conception is ~38 weeks.
Ultrasound Method
Measure gestational age at scan; EDD = scan date + (280 - GA in days). Best before 14 weeks.
Expert Tips
Terminology Comparison
| Term | Weeks | Description |
|---|---|---|
| Preterm | <37 | Before 37 weeks |
| Early Term | 37-38 | Baby considered early term |
| Full Term | 39-40 | Optimal delivery window |
| Late Term | 41 | 1 week past due |
| Post-Term | 42+ | 2+ weeks past due |
Frequently Asked Questions
How are due dates calculated?
Due dates are typically calculated using Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period. For cycles longer or shorter than 28 days, the formula adjusts by adding (cycle length - 28) days.
How accurate is a due date?
Only about 5% of babies are born on their exact due date. Most arrive within two weeks before or after. First-trimester ultrasound dating is accurate to ±5-7 days, while LMP-based dating can vary by ±2 weeks for irregular cycles.
What is Naegele's rule?
Naegele's rule, developed in the early 1800s, states that the expected due date equals the first day of the last menstrual period plus 280 days. It assumes a 28-day cycle with ovulation on day 14.
Why do some babies arrive early or late?
Full-term pregnancy spans 37-42 weeks. Factors affecting delivery timing include genetics, placental function, maternal health, and fetal development. Early term is 37-38 weeks; late term is 41 weeks; post-term is 42+ weeks.
When is ultrasound dating most accurate?
First-trimester ultrasound (before 14 weeks) provides the most accurate dating, typically within ±5 days. Crown-rump length measurements are most reliable. Later ultrasounds have ±2-3 week variance and are not used to change due dates.
Does cycle length affect due date calculation?
Yes. For cycles longer than 28 days, add the extra days to the due date. For shorter cycles, subtract. Example: a 35-day cycle adds 7 days. Irregular cycles reduce accuracy; ultrasound dating is recommended.
Key Statistics
Official Data Sources
⚠️ Disclaimer: This calculator is for educational and informational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider for pregnancy dating and prenatal care.
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