Enter the first day of your last menstrual period (LMP) to estimate your due date using Naegele's rule.
A last menstrual period beginning 2 March 2026.
Estimated due date 7 December 2026.
Counting from the LMP builds in an assumption about the cycle: a 28-day cycle with ovulation around day 14. If your cycles are consistently longer or shorter than 28 days, the estimate shifts by roughly the same number of days as the difference — a regular 35-day cycle typically pushes the date about a week later than this calculation shows. Cycles that are irregular make the LMP method less reliable still, which is exactly the situation where a dating scan matters most.
Births cluster around the estimate rather than landing on it, and arriving in the weeks either side of the date is ordinary. The practical value of the number is in scheduling care and tracking gestational age, not in predicting a day. Treating it as a deadline tends to cause more anxiety than it resolves.
An ultrasound in the first trimester measures the pregnancy directly and is generally used to set or confirm dating, replacing the LMP estimate when the two disagree by more than a set margin. If you have been given a date by a midwife or doctor, use theirs. This calculator is a convenience for the period before that appointment, or for a quick check afterwards.
The remaining time is calculated from today to the due date and shown as whole weeks plus leftover days, so it changes every day you reload the page. Once the date has passed the tool simply says so rather than counting into negative numbers.
By adding 280 days — 40 weeks — to the first day of the last menstrual period. That count starts about two weeks before conception, which is why gestational age is roughly two weeks ahead of the age of the embryo itself.
The estimate shifts. The LMP method assumes ovulation around day 14, so a consistently longer cycle means later ovulation and a later date, and a shorter cycle the reverse — roughly day for day. A first-trimester scan avoids the assumption entirely by measuring the pregnancy directly.
It is an estimate of a midpoint, and births spread out around it. It is most useful for tracking gestational age week by week and for scheduling appointments. Any decision that depends on precise dating should rest on clinical assessment rather than on this arithmetic.
Only approximately, and this tool does not do it. Working back 280 days from a due date returns the assumed LMP, and conception is generally taken as roughly two weeks after that under the same 28-day assumption. Because the assumption is the weakest part of the method, a conception date derived this way carries the same uncertainty as the due date, plus more.
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