Naegele's rule, and what it assumes
The standard method for estimating a due date is nearly two centuries old. Take the first day of the last menstrual period and add 280 days — forty weeks.
That is it. There is no adjustment for age, height, ethnicity, whether it is a first pregnancy, or anything else. The rule is a single addition, and it is what almost every due date in the world is built from.
It rests on two assumptions, and both are averages rather than facts about any individual:
A 28-day cycle. Cycle length varies substantially between people and between cycles in the same person. A 35-day cycle is entirely normal and is not what the rule expects.
Ovulation on day 14. Even in a 28-day cycle, ovulation does not reliably land on day 14. Studies tracking hormonal markers find it spread across a range of days, and the variation is larger in longer cycles.
The tool corrects for the first of these — enter your usual cycle length and it shifts the date by the difference from 28. It cannot correct for the second, because nobody can without measurement.
If your cycles are irregular, last-period dating is unreliable in a way no adjustment fixes. That is one of the situations where an early scan matters most.
Why you are "four weeks pregnant" two weeks after conception
This confuses almost everyone the first time, and it is not a mistake.
Gestational age is counted from the first day of the last period — which is roughly two weeks before conception happened. So the clock starts before the pregnancy does.
The consequences follow directly:
- At conception you are already counted as two weeks pregnant.
- A positive test around the time of a missed period puts you at about four weeks.
- The "forty weeks" of pregnancy contains around thirty-eight weeks of actual gestation.
The convention exists because the last period is a date most people know, and conception usually is not. It trades biological precision for something you can actually report at a first appointment, and every clinical guideline, growth chart and scan measurement is calibrated to it. It is the shared reference point, not a claim about biology.
The due date is the middle of a range
This is the most important thing on the page, and the thing a single bolded date on a screen actively works against.
About 4 percent of babies are born on their estimated due date. Roughly two thirds arrive within a week either side. The rest are spread more widely still.
A 2013 study by Jukic and colleagues is the clearest evidence, because it used pregnancies where ovulation had been pinpointed by hormone measurement rather than estimated. Even with that precision — removing the guesswork about cycle length and ovulation timing entirely — the length of gestation still varied by as much as five weeks between individuals.
That variation is not measurement error. It is real biological difference in how long pregnancies last.
So the honest way to read the output above is as the centre of a distribution. The tool shows the week either side because that band contains most births, and because "sometime in the fortnight around the 14th" is a truthful answer where "the 14th" is not.
Practically: this is why waiting past a due date is normal rather than a problem, and why first-time pregnancies in particular tend to run slightly longer.
The scan overrides the calculation
If you take one clinical point from this page: a first-trimester ultrasound dates a pregnancy more accurately than any arithmetic from a last period, and where the two disagree, the scan wins.
ACOG's guidance is explicit. Measurement of the crown-rump length in the first trimester is the most accurate method of establishing gestational age, and the ultrasound-derived date should replace the period-based one when the difference exceeds a defined margin.
That margin is deliberately tight early and looser later, because ultrasound dating itself degrades as pregnancy progresses. In the first trimester the threshold is around five to seven days; by the third trimester it is three weeks or more. The reason is biological: embryos in early pregnancy grow at very similar rates, so size predicts age well. Later on, genuine size differences between healthy babies swamp the signal.
The practical consequence is that a due date established at an early scan should not be revised at a later one. Once dating is set, it is set.
If your scan gave you a different date from this calculator, use the scan's.
IVF dating is different, and better
Where conception was assisted, the largest source of uncertainty disappears: the date of fertilisation is known rather than inferred.
The due date is counted from the transfer, adjusted for how far the embryo had already developed:
- Day-3 transfer: due date is 263 days from transfer.
- Day-5 blastocyst transfer: 261 days from transfer.
Both work back to the same 266-days-from-fertilisation figure, less the days already elapsed. There is no cycle length to estimate and no ovulation to guess at.
This is why IVF due dates are treated as more reliable than scan dates and are generally not revised by ultrasound at all. The known date beats a measurement.
What "term" actually means
The vocabulary was tightened in 2013, because "term" had been covering a five-week span as though every week in it were equivalent. It is not.
- Early term — 37 weeks 0 days to 38 weeks 6 days
- Full term — 39 weeks 0 days to 40 weeks 6 days
- Late term — 41 weeks 0 days to 41 weeks 6 days
- Post-term — 42 weeks 0 days and beyond
The change was made because outcomes at 37 weeks are measurably worse than at 39 — small differences in respiratory problems and other measures, but consistent ones. That matters when a delivery is being scheduled rather than happening on its own, and it is why elective deliveries before 39 weeks are now discouraged without a medical reason.
The tool shows the date each of these thresholds falls on, because they are more informative than the single due date and rarely presented.
Working backwards to a conception date
The tool can estimate one, and it is worth being careful about what that estimate is and is not.
Working backwards assumes ovulation happened at a typical point in the cycle. That is an average across a population, not a fact about a particular cycle, and the uncertainty runs to several days in either direction.
Two further points that get overlooked:
Intercourse and fertilisation are not the same date. Sperm remain viable in the reproductive tract for up to about five days, so conception can occur days after.
Implantation is later still, typically six to twelve days after fertilisation, which is why a pregnancy test does not turn positive immediately.
A calculated conception date is therefore approximate by several days at best. It is fine for satisfying curiosity. It is not a suitable basis for any conclusion that depends on precision, and if such a question genuinely matters, it needs a clinician and probably a scan rather than arithmetic.
What this page is not
These are estimates produced by published conventions, shown with their limits stated. Nothing here is medical advice, nothing here replaces antenatal care, and no calculator can tell you whether a pregnancy is progressing normally.
If a date from this page conflicts with one from a midwife, doctor or scan, theirs is the one to use. They have information this page does not — a measurement, an examination, and your history.