How your due date is calculated
This calculator uses Naegele's rule — the standard method most doctors use for a first estimate: adding 280 days (40 weeks) to the first day of your last menstrual period. If your cycle is longer or shorter than the typical 28 days, the estimate adjusts accordingly, since ovulation timing shifts with cycle length.
Why "due date" is an estimate, not a deadline
Only a small percentage of babies are actually born on their exact due date. Full-term birth is considered anywhere from 37 to 42 weeks, so the date here is best understood as the middle of a several-week window rather than a fixed appointment.
Trimesters at a glance
First trimester: weeks 1–13, when major organs begin forming. Second trimester: weeks 14–27, often called the most comfortable stage, with noticeable growth and movement. Third trimester: weeks 28–40, focused on rapid weight gain and final development before birth.
Frequently asked questions
Is this calculator accurate if I don't know my exact LMP date? An ultrasound dating scan (usually done in the first trimester) is generally more accurate than LMP-based calculation alone, especially for irregular cycles.
Can the due date change during pregnancy? Yes — your doctor may adjust it based on ultrasound measurements, particularly if there's a meaningful difference from the LMP-based estimate.