Due Date Calculator
From last period to anticipated birthday: the estimated due date, the conception window, and how far along you are now.
Last updated: August 5, 2026
How a due date is estimated
The classic method adds 280 days (40 weeks) to the first day of your last menstrual period. That assumes a regular 28-day cycle, where ovulation and conception land around day 14. The pregnancy length, counted from the LMP, is why the "40 weeks" number includes about two weeks before you were actually pregnant.
For cycles shorter or longer than 28 days, the calculator shifts the due date by the difference, matching the day ovulation most likely fell. It also works backwards from the due date to estimate the likely conception window.
The approach
Worked example
Last period began 10 March 2026, regular 28-day cycle.
Due date ≈ 15 December 2026. Conception window around 24 March.
The doctor will likely confirm with an ultrasound, which can date the pregnancy more precisely — but this gives you the milestone figure to start planning around.
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Frequently asked questions
What if I don't know my LMP?
An ultrasound in the first trimester is the most reliable way to date a pregnancy when the LMP is unknown or irregular. This calculator works well from a known LMP on a regular cycle.
Why are the weeks 40 but development closer to 38?
Because pregnancy is measured from the last menstrual period, not from conception. Subtracting the ~2 weeks before ovulation gives the 38-week fetal gestational age from conception.
Is the trimestar split fixed?
There is no universal definition, but a common split is: first trimester ends at week 13, second at week 27, and the third from week 28. The calculator uses that standard.
This is an estimate only and not medical care. Confirm your dating with an obstetrician or midwife, who will account for ultrasound findings and your history.