Due Date Calculator · Practical guide
How to Calculate Your Due Date from Your Last Period
One rule covers it: first day of your last period plus 280 days — the same starting point your provider uses.
At a glance
The Rule in One Sentence
Take the first day of your last menstrual period (the day bleeding started) and add 280 days — exactly 40 weeks. That date is your estimated due date under Naegele's rule, the standard obstetric formula, assuming a 28-day cycle.
Interactive tool
Due Date Calculator
Enter your own values, then compare the result with the examples below.
Trimester Dates
| Trimester | Start Date | End Date |
|---|---|---|
| First Trimester | -- | -- |
| Second Trimester | -- | -- |
| Third Trimester | -- | -- |
The Mental-Math Version
Naegele's rule has a calendar shortcut: add 1 year, subtract 3 months, add 7 days. For an LMP of January 10, 2026: January 10, 2027 minus 3 months is October 10, 2026, plus 7 days gives October 17, 2026 — exactly 280 days later. The calculator above does this instantly and also shows your conception window and trimester dates.
Due Date Lookup for a 28-Day Cycle
| First Day of Last Period | Estimated Due Date |
|---|---|
| January 1, 2026 | October 8, 2026 |
| February 1, 2026 | November 8, 2026 |
| March 1, 2026 | December 6, 2026 |
| April 1, 2026 | January 6, 2027 |
| May 1, 2026 | February 5, 2027 |
| June 1, 2026 | March 8, 2027 |
How Confident Should You Be?
A due date is the center of a window, not an appointment. Only about 4-5% of babies arrive on their exact due date, though the large majority come within two weeks either side. A first-trimester ultrasound, accurate to about 5-7 days, is the most precise dating method, and providers usually revise the estimate if it disagrees with the LMP date by more than a week.
Frequently Asked Questions
Do I count from the first or last day of my period?
Always the first day — the day bleeding started, not when it ended. Naegele's rule and every clinical dating method count from day one of the last menstrual period.
What if my cycle is not 28 days?
Add or subtract one day for every day your cycle differs from 28. A 35-day cycle pushes the due date 7 days later; a 24-day cycle moves it 4 days earlier. The calculator adjusts for this automatically.
Why is pregnancy 40 weeks if it is called 9 months?
Forty weeks is counted from the last period, which is about two weeks before conception. From conception the average pregnancy is 38 weeks — closer to nine calendar months.
Is the due date different for IVF pregnancies?
IVF dating uses the embryo transfer date and embryo age instead of the LMP, which is more precise. Your clinic provides the official date; LMP-based calculators are approximations for natural conception.