A due date calculator gives expecting families their estimated delivery date — the anchor around which prenatal care, parental leave, and baby preparations are scheduled. Using the first day of your last menstrual period (LMP), it estimates the date at 280 days (40 weeks) of gestation, the most widely used clinical method.
While the headline date is the big number, this calculator also shows the gestational week you are in, your progress through trimesters, and the normal delivery window (38-42 weeks). Understanding how the estimate works removes anxiety when the due date comes and goes — because a "late" date within that window is entirely normal.
How a Due Date Is Estimated
The classic estimate begins at the LMP and adds 280 days. Naegele's rule expresses it conveniently: take the first day of your last period, add 7 days, subtract 3 months, and add a year. For an LMP of March 10, the due date estimates December 17 of the following calendar year.
The 280-day convention assumes an average cycle length of 28 days with ovulation on day 14. For cycles that differ, adjusting may help: the estimate can be shifted by the difference between a longer cycle and 28 days. But the LMP method remains the practical first step for almost every pregnancy.
Because the LMP is usually the most certain and universally recorded date, clinicians anchor initial dating to it, then validate with ultrasound — typically a crown-rump measurement between 8 and 13 weeks, which is the most precise dating tool available and may revise the date if the discrepancy exceeds a week.
The Accuracy and Meaning of Your Due Date
Fewer than 5% of babies arrive exactly on the estimated due date. Statistics from large birth databases show most births occur in the 38-40 week window, with a normal range defined as 37-42 weeks. Carrying to 41 weeks with a monitored pregnancy is common and generally safe; induction is usually considered at 41+ weeks.
Preterm delivery is defined as birth before 37 weeks and requires special care; postterm is birth after 42 weeks. Because the range is wide, parents are best served by treating the due date as a planning point rather than a deadline — packing the hospital bag earlier rather than later (around 36 weeks) is the practical takeaway.
Dating accuracy matters most for medical decision-making. An accurate early ultrasound date helps providers assess growth over time, time genetic screening, and determine whether a post-dates pregnancy warrants intervention. The first-trimester ultrasound remains the gold standard for that reason.
Adjusting for Irregular Cycles and IVF
For women with regular cycles, LMP dating works well. For short cycles (21-25 days) or long cycles (35+ days), ultrasound is more reliable because the timing of ovulation — and therefore conception — varies more than 28-day convention assumes. Irregular, missing, or unknown LMP dates also push providers to ultrasound-based dating.
IVF pregnancies set the date precisely: for a day-5 blastocyst transfer, gestational age is the transfer date plus 19 days (or 266 days from conception, i.e., the transfer day for a fresh day-3 transfer plus 17 days). This removes virtually all dating uncertainty and is documented in the fertility clinic's records.
Wherever your due date originates, the calculation serves the same purpose — planning care, tests, and timing. Record your LMP, let ultrasound refine the number in the first trimester, and treat subsequent due-date changes as standard practice rather than cause for alarm.
The due date is a meticulously calculated estimate that anchors prenatal care — not a precise appointment. Use this calculator with your LMP to get the date, gestational age, and trimestrial progress, and expect a first-trimester ultrasound to refine it. Understand the 38-42 week normal window and use your date as a roadmap for planning, while leaving generous room for your baby's own timing.