What is the Pregnancy Due Date Calculator?
Your estimated due date is the anchor around which all pregnancy planning revolves — prenatal appointments, genetic testing windows, maternity leave applications, nursery preparation, and birth plan decisions all depend on knowing when to expect your baby. Our Pregnancy Due Date Calculator uses Naegele's Rule — the same standard method used by obstetricians worldwide — to calculate your EDD from your last menstrual period. It also shows your current gestational week and trimester, and lists key milestone dates throughout your pregnancy. If you know your conception date instead of your LMP, you can use that as your starting point for an equally accurate calculation.
Why Use This Calculator?
- Find your estimated due date based on last menstrual period (LMP) or conception date
- Know your current pregnancy week and trimester instantly
- See key milestone dates (end of first trimester, viability milestone, etc.)
- Understand the difference between gestational age and embryonic age
- Free, instant, and widely used method matching OB-GYN calculations
How to Use the Pregnancy Due Date Calculator
- Enter the Date of Your Last Menstrual Period (LMP) — or
- Enter your Conception Date (if known)
- Enter your Average Cycle Length (default 28 days)
- Click Calculate to see your estimated due date and current week
Formula & Methodology
Naegele's Rule (standard OB method): EDD = LMP + 280 days (40 weeks) Or: EDD = LMP + 9 months + 7 days
Adjusted for cycle length: EDD = LMP + 280 + (Cycle Length − 28) days
From conception date: EDD = Conception Date + 266 days (38 weeks from conception)
Example: LMP = September 1, 2025 (28-day cycle): EDD = September 1 + 280 days = June 8, 2026 Current week as of June 6, 2026: Week 39, Day 5
Real-Life Examples
- Standard LMP calculation: With a last menstrual period date of March 1 and a typical 28-day cycle, the estimated due date is approximately December 6 (40 weeks later).
- Longer cycle adjustment: For someone with a 32-day cycle instead of 28, the estimated due date shifts about 4 days later than the standard calculation would suggest.
- Dating scan confirmation: If a first-trimester ultrasound estimates gestational age slightly differently than the LMP-based calculation, most providers use the ultrasound date as the more reliable estimate.
How to Interpret Your Results
The estimated due date marks the middle of a normal delivery window, not a fixed deadline — most healthy pregnancies deliver anywhere from 37 to 42 weeks. Use this as a planning reference and confirm timing with your healthcare provider, especially if your cycle length differs from the standard 28 days.
Benefits
- Aligns with how doctors calculate gestational age so your results match your OB's records
- Helps plan maternity leave application timing
- Identifies the current trimester for appropriate prenatal care and activity guidelines
- Useful for tracking when specific prenatal tests are recommended
- Helps family members and employers understand the expected arrival date
Common Mistakes to Avoid
- Assuming the calculated due date is exact rather than an estimate — only about 5% of babies are born on their exact due date.
- Not adjusting for cycle length when it's notably longer or shorter than the standard 28 days assumed by the calculator.
- Treating this calculator as a substitute for a dating ultrasound, which is generally more accurate, especially with irregular cycles.
- Confusing gestational age (counted from LMP) with fetal age (counted from conception), which differ by about 2 weeks.
Tips for Best Results
- Use this estimate as a helpful planning reference, then confirm timing with a healthcare provider and dating ultrasound.
- If your cycle isn't close to 28 days, mention this to your provider, since it affects due date accuracy.
- Remember that a 'due date' represents the middle of a normal delivery window (roughly 37-42 weeks), not a fixed deadline.
References
- WHO — Preterm Birth: Definitions and Gestational Age Standards (WHO)
- NHS — Calculating Your Due Date (NHS UK)
- American College of Obstetricians and Gynecologists (ACOG) — Methods for Estimating the Due Date (ACOG Committee Opinion 700)
Frequently Asked Questions
Why is pregnancy measured from the last period, not conception?
Ovulation and conception typically occur about 2 weeks after the LMP. Since most people know their last period date but not their exact conception date, doctors use LMP as a consistent starting reference. This means at the moment of conception, you are already considered "2 weeks pregnant."
How accurate is the due date calculation?
The EDD is an estimate — only about 4% of babies are born on their exact due date. Most births occur between 38–42 weeks. First-trimester ultrasound (crown-rump length measurement) is the most accurate method for dating pregnancy, often refining the LMP-based estimate.
What is the difference between gestational age and embryonic age?
Gestational age is counted from the LMP and is used in all medical documentation (40 weeks total). Embryonic age is counted from conception and is 2 weeks shorter (38 weeks total). When doctors say "you are 12 weeks pregnant," they mean 12 weeks gestational age.
What are the three trimesters?
First trimester: Weeks 1–12 (organ development, highest miscarriage risk). Second trimester: Weeks 13–26 (fastest fetal growth, typically most comfortable for mother). Third trimester: Weeks 27–40 (rapid weight gain, birth preparation).
What does it mean if my due date changes after an ultrasound?
If the ultrasound-measured fetal size significantly differs from the LMP-based estimate (by more than 5–7 days in the first trimester), your provider may adjust the due date to match ultrasound findings. This is normal and the adjusted date is considered more accurate.
Why did my provider give me a different due date than this calculator?
Providers often use a dating ultrasound, which can be more accurate than a last-menstrual-period calculation, especially for those with irregular cycles. If the two dates differ by more than about a week, the ultrasound date is typically considered more reliable.
What does it mean if I go past my due date?
It's common — many healthy pregnancies extend a week or more past the estimated due date. Your provider will typically monitor you more closely as you approach 41-42 weeks, but going a few days past the estimate is not unusual.
Conclusion
Our Pregnancy Due Date Calculator gives you an accurate estimated due date, current pregnancy week, trimester, and key milestone dates based on your last menstrual period. Use it to plan your pregnancy journey from the very first week.
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