Due date calculator
Enter the first day of your last period and your usual cycle length to get an estimated due date, a likely conception date and the main pregnancy milestones.
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Processing and privacy: The result is calculated inside your browser and your data is not sent to any server.
How to use it
- Enter first day of your last period and average cycle length.
- The result appears straight away and updates as you edit.
- Copy the result, or copy a link that keeps the same values to share it.
How it is calculated
Due date = first day of last period + 280 days + (cycle length - 28). Likely conception = last period + cycle length - 14. The second trimester starts at 14 weeks, the third at 28 weeks and early term at 37 weeks.
Example
Inputs: First day of your last period = 2026-01-01, Average cycle length = 28 days. Result: Estimated due date: Thursday, October 8, 2026 (2026-10-08); Likely conception date: Thursday, January 15, 2026 (2026-01-15); Second trimester starts (14 weeks): Thursday, April 9, 2026 (2026-04-09).
Limits and caveats
- It assumes ovulation about 14 days before the next period, which varies between women.
- An early ultrasound date is more accurate; if they differ, follow the date your doctor sets.
- Only about 4% of babies arrive on the due date itself.
Questions about this tool
What is Naegele's rule?
Add 7 days to the first day of the last period, subtract 3 months and add a year, which is about 280 days. This tool adds exactly 280 days so February does not shift the answer.
Why does cycle length change the date?
The rule assumes a 28-day cycle. With a longer cycle ovulation usually comes later, so the due date moves by the extra days.
Is it normal to give birth before the due date?
Yes. Birth from 37 weeks counts as term, and most babies arrive within two weeks either side of the date.