Pregnancy Due Date Calculator
Estimated due date from your last menstrual period or conception date, with trimester breakdown
Frequently Asked Questions
Estimating a pregnancy due date from last menstrual period (LMP) or from a known conception date follows standard obstetric dating conventions, and — critically — these two starting points require different calculations, since LMP and actual conception date are roughly two weeks apart, not the same reference point.
Why LMP-based and conception-based dating give different starting calculations
The standard, most commonly used due-date convention counts 280 days (40 weeks) forward from LMP — this is the default most healthcare providers use, since LMP is typically easier for someone to know precisely than an exact conception date. If conception date is instead known precisely (through fertility tracking or IVF, for instance), the due date is calculated as 266 days (38 weeks) forward from that conception date — a different number of days because conception itself typically occurs roughly two weeks after LMP, so the two methods, correctly applied, converge on the same actual due date despite starting from different reference points and using different day counts. Using the LMP day-count (280 days) starting from an actual conception date, or vice versa, introduces the same roughly two-week error discussed in the Conception Date Calculator.
A worked example
The same actual pregnancy, dated correctly from LMP (280 days forward) and correctly from a known conception date (266 days forward), converges on the same estimated due date — but using the wrong day-count formula for whichever starting point is actually known (LMP versus conception) would shift the estimate by roughly two weeks in one direction or the other.
How this connects to your other pregnancy-related calculations
This calculator's LMP-based dating logic is the forward-direction companion to the Conception Date Calculator, which works backward from a due date to estimate conception, and to the Ovulation Calculator, which estimates the fertile window using the same underlying cycle-dating conventions.
Common mistakes
Applying the LMP-based 280-day count to an actually-known conception date (rather than the conception-specific 266-day count), or vice versa, is the most common dating error — always confirm which specific reference point (LMP or conception) is actually being used as the starting point before applying either day-count convention, since mixing them up shifts the estimate by roughly two weeks.