Pregnancy Calculator · Practical guide
How to Calculate Your Due Date From Your Last Period
Naegele's rule turns the first day of your last period into an estimated due date in one step.
At a glance
The 280-Day Rule
Your estimated due date is the first day of your last menstrual period plus 280 days (40 weeks). The classic mental shortcut — Naegele's rule — says the same thing: take the LMP, add one year, subtract three months, and add seven days. If your LMP started June 1, 2026, your due date is March 8, 2027.
Interactive tool
Pregnancy Calculator
Enter your own values, then compare the result with the examples below.
Milestones
| Week | Milestone | Date |
|---|---|---|
| Week 8 | Heartbeat detectable | -- |
| Week 12 | End of first trimester | -- |
| Week 20 | Anatomy scan | -- |
| Week 27 | Start of third trimester | -- |
| Week 40 | Estimated due date | -- |
A Worked Example
Everything the calculator reports flows from that single date. For an LMP of June 1, 2026:
| Milestone | Timing | Date |
|---|---|---|
| Estimated conception | LMP + 14 days | June 15, 2026 |
| Heartbeat detectable | Week 8 | July 27, 2026 |
| Second trimester begins | Week 13 | August 31, 2026 |
| Anatomy scan | Week 20 | October 19, 2026 |
| Third trimester begins | Week 27 | December 7, 2026 |
| Estimated due date | Week 40 | March 8, 2027 |
Adjusting for Cycle Length
The rule assumes a 28-day cycle with ovulation on day 14. If your cycles are reliably longer or shorter, shift the due date by the difference: a 35-day cycle moves the example due date to March 15, 2027, while a 24-day cycle pulls it earlier to March 4, 2027.
How Accurate Is the Estimate?
Treat the due date as the middle of a likely window, not an appointment. Only about 5% of babies are born on their exact due date, and a normal full-term delivery can happen anywhere from 37 to 42 weeks. If an early ultrasound disagrees with your LMP date by more than about a week, your provider will often re-date the pregnancy from the ultrasound.
Frequently Asked Questions
What is Naegele's rule?
It is the standard due-date formula: last menstrual period plus 280 days, equivalent to adding one year, subtracting three months, and adding seven days. It assumes a 28-day cycle with ovulation on day 14.
Can I get a due date without knowing my last period?
Yes. Use the due date your provider gave you, or ask about an early ultrasound — a first-trimester measurement can date a pregnancy to within about five to seven days.
Why did my doctor change my due date?
When an early ultrasound measurement disagrees with LMP-based dating by more than roughly a week — common with irregular cycles — providers often re-date the pregnancy from the ultrasound, which is more precise early on.
Is the due date the day I will actually deliver?
Usually not. Only about 5% of babies arrive on the exact date; most arrive within the surrounding weeks, and anything from 37 to 42 weeks is considered a normal full-term window.