Due Date Accuracy: What the Data Shows

How Often Are Due Dates Right?

It depends on what “right” means. If right means the exact calendar day, due dates are right only about 5% of the time — roughly 1 birth in 20. If right means landing inside a reasonable window, they perform far better: the large majority of births fall within two weeks of the estimate in either direction.

These figures describe population patterns, not your pregnancy — but they explain why providers treat the due date as a midpoint. A date that “misses” by a week is not a failed estimate; it is an estimate behaving exactly as the statistics say it will. For a plain-language walkthrough of what the estimate means for you personally, see our parent-friendly overview.

What Research Says About Gestation Length

The most informative studies are the ones that remove the biggest source of noise: uncertain dating. In a widely cited study published in the journal Human Reproduction, researchers followed women with precisely known ovulation dates and measured the time to birth. The average was about 268 days from ovulation — but the spread ran from 247 to 284 days across healthy, normal pregnancies. That is a 37-day range with no pathology involved at all.

The standard deviation was roughly 10 days, which is a statistician’s way of saying that about two-thirds of births fall within 10 days of the average, and about 95% fall within 20 days. This bell-curve shape is the fundamental reason due dates behave the way they do: the estimate sits at the center of a genuinely wide distribution, and no refinement of the formula can shrink the underlying biology.

Researchers have also probed what stretches or shortens the curve. Studies estimate that each pregnancy carries its own tempo, influenced by genetics — mothers and daughters, for instance, show correlated gestation lengths — and by factors like parity and fetal sex, though these effects are measured in days, not weeks. The takeaway is consistent across the literature: gestation length is a biological variable with a wide normal range, and the due date is the center of that range, nothing more.

Why 280 days if the average is 268 from ovulation?

The classic 280-day figure counts from the first day of the last menstrual period and assumes ovulation on day 14, adding roughly two pre-conception weeks to the 268-day average. The two numbers describe the same pregnancies from different starting lines. Neither is more “correct” — they are just measured from different day zeros.

Factors That Shift Due Date Accuracy

Not all estimates are created equal. Some pregnancies are inherently easier to date than others, and the same dating method can be sharp in one case and fuzzy in another. These are the factors that move the needle most:

Cycle length: the quiet dating disruptor

The standard formula assumes a 28-day cycle with ovulation on day 14. If your cycles run 35 days, you likely ovulated around day 21 — a full week later than the formula assumes — and an unadjusted LMP date will land about a week early. Adjusting the estimate by the difference (cycle length minus 28) fixes most of this error. Irregular cycles compound the problem, because ovulation timing becomes unpredictable; that is when ultrasound dating earns its keep.

Ultrasound timing: earlier beats later

First-trimester embryos grow at remarkably consistent rates, which is why a crown-rump length measurement dates a pregnancy to within about 5 days. Later in pregnancy, individual growth differences widen, and dating from size becomes progressively less reliable. A scan at 10 weeks is a dating tool; a scan at 30 weeks is a growth check. Confusing the two is a common source of unnecessary worry when a late scan “measures ahead or behind.”

Dating Methods Compared: LMP vs Conception vs Ultrasound vs IVF

Every dating method answers the same question — when did this pregnancy effectively begin? — but they start from different evidence. Here is how they stack up on accuracy and when each one is the right tool:

LMP dating: the workhorse

Dating from the last menstrual period (Naegele’s rule: add 280 days to the LMP) remains the default because it needs no equipment — just a calendar and a remembered date. Its weakness is the day-14 ovulation assumption. With regular 28-day cycles and a certain LMP, it is respectably accurate; with irregular cycles, it is the method most likely to be off by weeks.

Conception dating: precise start, fuzzy middle

If you know your ovulation date — from fertility tracking, for example — adding 266 days gives a solid estimate. The residual error comes from implantation and early development, which vary by a few days between pregnancies. It beats uncertain LMP dating but cannot match a first-trimester scan.

IVF dating: the gold standard

With embryo transfer, the embryo’s age is known exactly: a day-5 blastocyst transferred today is treated as 19 days gestational age (14 days + 5 days of embryo age), so the due date is the transfer date plus 261 days. This is the most precise dating method in existence — which makes it all the more telling that IVF babies still arrive across the same wide window. Precision in dating does not buy precision in birth timing.

The Term Spectrum: 37 to 42 Weeks

Because due dates are midpoints, medicine needed vocabulary for the whole window — and in 2013 the definitions were sharpened to reflect what the data showed about newborn outcomes. The current breakdown, used by ACOG and most providers worldwide:

The estimated due date — 40 weeks 0 days — sits inside the full-term band, not at its edge. That placement is deliberate: it reflects the evidence that outcomes are generally best in the 39-to-40-week range. Births at 37–38 weeks are still “term” but carry slightly higher rates of minor newborn issues, which is why elective deliveries are not scheduled before 39 weeks without a medical reason.

Understanding this spectrum reframes the accuracy question entirely. A baby born at 38 weeks 3 days or 41 weeks 1 day did not defeat the estimate — both land inside the medically normal range the estimate was built to describe. Only arrivals before 37 weeks (preterm) or at/after 42 weeks (post-term) fall outside it, and those are the cases your provider actively manages.

What the Numbers Mean for Your Planning

Statistics are only useful if they change what you do. Here is the practical translation: treat your due date as the center of a four-week planning season — two weeks before, two weeks after. That single shift in thinking covers roughly 85% of births and eliminates most of the anxiety the exact date creates.

Practically, that means having the hospital bag packed and logistics settled by 37 weeks, not 40. It means not scheduling anything immovable — flights, nonrefundable trips, major work deadlines — for the due-date week itself. And it means telling well-meaning relatives that “the window” opens around 38 weeks, which buys you a surprising amount of peace in the final stretch.

For the care side, the numbers explain your provider’s playbook: routine monitoring through 40 weeks, closer surveillance and an induction conversation around 41, and a general recommendation for induction by 42. Each step maps to where the statistical risks begin to tilt — patience while the odds favor waiting, action when they start to favor delivery. If you want to follow the journey week by week, the week-by-week guide tracks what is happening at each stage of that window.

Honest Limits of the Data

Every statistic on this page describes populations, not individuals. A 5% on-the-date rate tells you nothing about whether your baby will arrive on Tuesday or three weeks from Tuesday — it only tells you that guessing the exact day is a losing game for everyone. Population data sets expectations; it does not make predictions.

The figures are also averages across diverse groups. Dating accuracy shifts with cycle regularity, ultrasound access and timing, BMI, and whether a pregnancy is singleton or multiple — so any single number is a blend of easier and harder cases. Your provider’s judgment, informed by your specific history and scans, will always beat a general statistic.

Finally, remember what no study can fix: the outcome being measured (the day labor begins) is partly random. Even a perfectly dated pregnancy, known to the day, delivers across a multi-week window. Honest uncertainty is not a flaw in the science — it is the science. The due date remains genuinely useful for scheduling care and milestones; it simply was never a crystal ball.

One more caveat worth stating plainly: most published figures come from high-resource settings with routine early ultrasound access, which may not reflect every pregnancy everywhere. Where dating starts later or relies on LMP alone, the real-world accuracy is likely a little worse than the textbook numbers suggest — another reason your provider’s individual assessment matters more than any population average.