Pregnancy Due Date Calculator
Estimate your due date based on your last menstrual period
What is it?
This calculator estimates your due date using Naegele's Rule, a standard obstetric formula that adds 280 days (40 weeks) to the first day of your last menstrual period (LMP).
Formula
Estimated Due Date = LMP + 280 days
Formula Explanation
Naegele's Rule assumes a typical 28-day menstrual cycle with ovulation around day 14, and pregnancy lasting about 280 days (40 weeks) from the first day of the last period — not from conception. It's a widely used clinical approximation rather than a precise prediction.
Example Calculation
If your last menstrual period started on January 1, adding 280 days gives an estimated due date of roughly October 8 of the same year.
How to Use
- Enter the first day of your last menstrual period.
- View your estimated due date, current week of pregnancy, and trimester guidance.
Benefits
- Gives an immediate estimate without needing an ultrasound.
- Shows current week of pregnancy and trimester at a glance.
- Useful starting reference to bring to your first prenatal appointment.
Use Cases
- Getting an early estimate before your first doctor visit.
- Tracking which trimester you are currently in.
- Planning ahead for maternity leave or major life events around the due date.
What Your Result Means
The estimated due date marks the average expected delivery date for a 40-week pregnancy counted from your LMP. It is a statistical average, not a guarantee — full-term delivery is considered anywhere from 37 to 42 weeks.
Tips
- Confirm this estimate with an ultrasound dating scan, which is generally more accurate.
- Track how weeks progress rather than fixating on the exact due date.
- Bring this estimate to your first prenatal visit for comparison.
Common Mistakes
- Assuming the due date is exact — only about 5% of babies arrive on their calculated due date.
- Using an irregular or inaccurate LMP date, which throws off the entire estimate.
- Not accounting for cycles longer or shorter than 28 days, which shifts actual ovulation timing.
FAQs
How accurate is this due date estimate?
Naegele's Rule provides a standard estimate, but only about 5% of babies are born exactly on their due date — actual delivery can occur anywhere from 37 to 42 weeks.
What if my cycle isn't 28 days?
This estimate assumes a standard 28-day cycle; if your cycle is significantly longer or shorter, an ultrasound-based estimate from your doctor will be more accurate.
When should I see a doctor?
Schedule your first prenatal appointment as soon as you know or suspect you're pregnant, regardless of this estimate.
Is due date calculated from LMP or conception?
Naegele's Rule calculates from the first day of your last menstrual period, not from conception, which typically occurs about 2 weeks later.
What counts as full term?
Full term is generally considered 39 to 40 weeks, with "early term" at 37-38 weeks and "late term" at 41 weeks — all within the normal range.
Can an ultrasound change my due date?
Yes, an early ultrasound (especially in the first trimester) is often considered more accurate than LMP-based dating and may adjust your due date.
Why does the estimate change if I re-enter the same LMP?
It shouldn't for the same LMP date — if the estimated due date stays fixed but 'weeks pregnant' changes, that's expected since it's calculated relative to today's date.
Does this work for irregular cycles?
Not reliably — Naegele's Rule assumes regular 28-day cycles, so irregular cycles make this estimate less accurate; a doctor-ordered ultrasound is recommended instead.
References
This tool is for educational purposes only. Not medical advice.
Last updated: July 25, 2026