WebToolsHUBFor general information only — this is not medical advice. Cycle length adjustments are approximations, and the dating from an ultrasound or your healthcare provider takes precedence.
How to estimate a due date
- Enter the first day of the last menstrual period (LMP).
- Adjust the cycle length if it differs from 28 days — longer cycles push the estimate later, shorter ones earlier.
- Read the estimated due date, calculated with Naegele’s rule (LMP plus 280 days, adjusted).
- If the pregnancy is underway, the tool shows the current gestational age in weeks and days plus a trimester progress bar.
- Note the milestone dates: end of first trimester, end of second trimester, and when early term begins.
Frequently Asked Questions
How does Naegele’s rule work?
It adds 280 days — 40 weeks — to the first day of the last menstrual period, assuming a 28-day cycle with ovulation on day 14. It is the standard paper method clinicians still use for a first estimate, before ultrasound dating refines it.
Why does cycle length change the estimate?
The rule assumes ovulation happened 14 days after the LMP. With a 32-day cycle ovulation likely happened around day 18, so the estimate shifts about 4 days later; a 26-day cycle shifts it 2 days earlier.
How accurate is a due date?
Treat it as a window, not a day: most births fall between 37 and 42 weeks, and only around 4-5% arrive exactly on the estimated date. First-trimester ultrasound dating is more precise than any LMP calculation.
Is my date information stored or uploaded?
No. The calculation runs locally in your browser from the date you enter, and nothing is saved, logged or transmitted — close the tab and nothing remains.
Is this medical advice?
No. This is a general-information estimate for planning only. Your midwife or doctor’s dating — especially after an ultrasound — takes precedence, and any concerns belong with your healthcare provider.