Pregnancy Due Date Calculator
Estimate your due date from the first day of your last period — plus how far along you are and which trimester you're in.
Your dates
28 days is the standard assumption — adjust if yours is longer or shorter.
Estimated Due Date
May 30, 2027
Sunday
- CCopy result
- DDownload PDF
- SSave this calculation
- HToggle history
- ?Show this panel
- EscClose open panel
Disabled while typing in a field.
Last updated: August 25, 2026 · Reviewed by the DoCalc team
How Due Dates Are Estimated
A pregnancy due date is almost always estimated using Naegele's rule — a simple, decades-old method based on the first day of your last menstrual period (LMP), not the date of conception itself. It's a statistical estimate of a 40-week pregnancy, not a prediction of the exact birth date.
The Formula
280 days assumes ovulation occurs on day 14 of a 28-day cycle. If your average cycle runs longer or shorter, ovulation (and so conception) shifts accordingly, which is why this calculator adjusts the estimate based on your actual cycle length.
Worth knowing: Only about 5% of babies are born on their exact estimated due date. Most arrive within roughly two weeks of it, before or after — treat the date as a planning midpoint, not a deadline.
Trimesters at a Glance
| Trimester | Weeks |
|---|---|
| First | 1-12 |
| Second | 13-26 |
| Third | 27-40+ |
Why "Weeks Pregnant" Starts Before Conception
Pregnancy is conventionally dated from the first day of the last period, not from conception — so by the time conception actually occurs (around week 2), you're already considered "2 weeks pregnant." This can feel counterintuitive, but it's the standard convention used by essentially every pregnancy tracker and healthcare provider, which is why this calculator uses it too.
Frequently Asked Questions
How is the due date calculated?
Using Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period, adjusted for a cycle length other than the standard 28 days.
How accurate is a due-date estimate?
Only about 5% of babies are born on their exact estimated due date — it's a statistical midpoint, not a prediction. Most babies arrive within about two weeks of it, before or after.
What if my cycle isn't 28 days?
Enter your actual average cycle length — a longer or shorter cycle shifts the estimated ovulation and due date accordingly, since Naegele's rule assumes a 28-day cycle by default.
When does each trimester start?
First trimester: weeks 1-12. Second trimester: weeks 13-26. Third trimester: weeks 27 onward, until birth around week 40.
Is this a substitute for a doctor's estimate?
No — an ultrasound-based estimate from a healthcare provider is more accurate than a last-period-based calculation, especially for irregular cycles. Use this as a quick estimate, not medical guidance.
Related calculators
Body Fat & Ideal Weight Calculator
Body fat, ideal weight, and daily water intake.
Calculate → InsuranceLife Insurance Needs Calculator
How much coverage your family would actually need.
Calculate → DirectoryBrowse All Calculators
Every calculator on DoCalc, organized by category.
Explore →