The arithmetic behind the estimate
The last-period method is Naegele’s rule, and it is refreshingly simple: add 280 days — forty weeks — to the first day of your last menstrual period. A last period starting January 1 gives a due date of October 8. The calculator adds one refinement: if your average cycle is longer or shorter than 28 days, the estimate shifts by the difference, because ovulation (and therefore conception) likely happened correspondingly later or earlier. With a 33-day cycle, that same January 1 period moves the estimate five days later, to October 13.
The conception-date method adds 266 days instead. The two are the same model viewed from different anchors: in a textbook 28-day cycle, conception around day 14 means a January 15 conception and a January 1 period both point to October 8. When you know the conception date precisely, that method skips the assumption about when ovulation happened — which is exactly the assumption that makes the period method wobble for irregular cycles.
Why week counting starts before pregnancy does
Gestational age is counted from the last period because it is the one date in early pregnancy that is usually known rather than inferred. The consequence surprises many people: during “week 1” and most of “week 2” there is no pregnancy yet, and a positive test around a missed period already puts you at about week 4. Every milestone in this calculator — the end of the first trimester at week 13, the anatomy scan window near week 20, the third trimester at week 28, full term at week 39 — is counted on this convention, as is essentially all prenatal care. If you mentally track from conception instead, you will run about two weeks behind every conversation with your provider.
A due date is a distribution, not a deadline
Only around one baby in twenty arrives on the estimated date itself; most arrive within a couple of weeks on either side. The estimate’s real job is to anchor everything else — scheduling the scans, timing screening tests that are only valid in specific week ranges, and defining what “early” or “overdue” means for this pregnancy. That is why an accurate anchor matters even though the birth date itself is unpredictable, and why a first-trimester dating ultrasound takes precedence over calendar math: early embryos grow at a remarkably consistent rate, so measuring one dates a pregnancy more directly than any recollection of dates can. If your provider re-dates the pregnancy after a scan, update your expectations — theirs is the better estimate, and this calculator will happily agree with it if you work backwards.
The milestone list deserves one note: “full term begins” at week 39 reflects the modern definition, under which weeks 37–38 are called early term. Babies born then usually do well, but the distinction exists because those extra days of development measurably matter — it is why elective deliveries are generally not scheduled before 39 weeks without a medical reason.
Getting the most out of this page
Use the cycle-length field honestly rather than leaving the default: if your cycles run 31 days, entering that moves the estimate three days — a small correction that keeps early scans and tests booked in their intended windows. If your cycles are too irregular for a meaningful average, use the calculator for the milestone timeline once your provider has dated the pregnancy, rather than for the initial estimate. And treat the gestational-age readout as the page’s most practically useful number: knowing you are “24 weeks, 3 days” answers the question every appointment, form, and pregnancy resource will ask.