How Is a Due Date Calculated? A Simple Guide

Every due date starts as a piece of arithmetic. To answer how is a due date calculated: a clinician adds 280 days (40 weeks) to the first day of your last menstrual period — a two-century-old shortcut called Naegele's rule — then confirms or replaces that date with a first-trimester ultrasound, the most accurate method, good to within about 5–7 days. The honest footnote nobody mentions: only about 4–5% of babies arrive on the due date itself, and most land within roughly two weeks on either side. Here is exactly how the date is set, when an ultrasound overrides your dates, and what that single day really means.

The starting point: Naegele's rule and the 280-day count

The classic method is named after Franz Naegele, a German obstetrician (1778–1851). The rule is simple: take the first day of your last menstrual period (LMP), subtract three months, then add seven days and one year. That works out to 280 days — the familiar 40 weeks — from the start of your last period. Our pregnancy due date calculator runs exactly this math for you.

Here is the quirk worth understanding: those 40 weeks are counted from your last period, not from conception. Naegele's rule assumes a textbook 28-day cycle with ovulation on day 14, so it bakes in about two weeks before you were actually pregnant. That is why you can be "four weeks pregnant" only a couple of weeks after conception. It is a clever, durable convention — but as a 2021 review in the Australian and New Zealand Journal of Obstetrics and Gynaecology put it, Naegele's rule is best treated as a guideline for the expected date, not a fixed appointment.

Counting from conception, ovulation, or an IVF transfer

If you happen to know your conception or ovulation date — from tracking, a known ovulation test, or timed intercourse — the count is 266 days (38 weeks) instead of 280. The 14-day difference is simply the pre-ovulatory stretch that Naegele's rule includes but conception dating does not.

IVF pregnancies are dated most precisely of all, because the embryo's exact age is known. The formula is the due date equals the transfer date plus (266 minus the embryo's age in days). In practice that means a day-3 (cleavage-stage) transfer adds 263 days, and a day-5 (blastocyst) transfer adds 261 days. You subtract the embryo's age because it was already several days old when it was transferred — its development did not start on transfer day. Even with this precision, a first-trimester ultrasound is still recommended to confirm the date and check for twins.

Why a first-trimester ultrasound is the gold standard

Before about 14 weeks, embryos grow at a strikingly predictable rate, so measuring the crown-rump length (the distance from head to bottom) pins down gestational age to within roughly 5–7 days. That is more reliable than most people's memory of their last period. As pregnancy advances, natural size differences widen — a second-trimester scan is accurate to about 10–14 days, and a third-trimester scan only to about 21–30 days — which is why the earliest scan wins.

The American College of Obstetricians and Gynecologists (ACOG Committee Opinion No. 700) sets clear rules for when the ultrasound date should replace the LMP date. If the two disagree by more than the threshold for that stage of pregnancy, the ultrasound date is used.

ACOG Committee Opinion No. 700 redating thresholds. A pregnancy without a confirming ultrasound before 22 weeks is considered "suboptimally dated." Once an early scan sets the date, later scans are not used to move it.

When your cycle isn't a textbook 28 days

Naegele's rule quietly assumes everyone ovulates on day 14 of a 28-day cycle. Many people don't. If your cycles run long — say 35 days — you probably ovulate later, so the plain LMP calculation can place your due date about a week too early; shorter cycles push it the other way. A rough fix is to add or subtract the difference from 28 (a 35-day cycle adds seven days), and a good calculator will do this once you enter your usual cycle length.

But if your cycles are genuinely irregular, if you conceived soon after stopping hormonal birth control, or if you simply are not sure of your last period, LMP dating becomes shaky — and that is exactly when an early ultrasound earns its keep. Background factors also nudge the length of a pregnancy: the review evidence links gestation length to height, parity, ethnicity, and pre-pregnancy weight. Knowing your starting point is useful context, and our BMI calculator and healthy-BMI guide can help you gauge it, though your prenatal team individualizes weight-gain targets.

The honest truth: almost nobody delivers on the due date

The due date is a single point at the center of a wide, normal spread — not a finish line. Only about 4–5% of babies are born on their exact due date, and the great majority arrive within roughly two weeks on either side of it. Even in the cleanest data, that spread is real: a 2013 study in Human Reproduction that tracked healthy natural pregnancies from a known ovulation date found the length of gestation varied by 37 days — more than five weeks — even after excluding preterm births and complications.

First pregnancies also tend to run a touch longer: a classic 1990 analysis in Obstetrics & Gynecology found first-time mothers typically ran about eight days — just over a week — past 40 weeks (a median, not an average). And U.S. birth data show most babies now arrive at 39–40 weeks, the full-term window. None of this means anything is wrong when the date comes and goes — it is simply how a population-level average behaves for any one person. Think "due month," not "due day."

Early, full, late, and post term: what the labels mean

Because the old catch-all word "term" lumped together 37 weeks and 42 weeks — which are not clinically the same — ACOG (Committee Opinion No. 579) split it into precise labels. They shape decisions like when an elective delivery is reasonable and when extra monitoring begins.

ACOG Committee Opinion No. 579 term definitions. This is why providers generally avoid non-medically-indicated deliveries before 39 weeks: full term carries the best outcomes.

How to think about your due date

Accurate dating matters for real reasons — it sets the timing of screening tests, growth checks, and any conversation about induction — which is why clinicians work to get it right early and then hold it steady. So take the number seriously as a planning anchor, but hold it loosely as a prediction. Your best estimate is a well-calculated midpoint of a normal, roughly month-wide range, and arriving "off" the date is the rule, not the exception.

In the meantime, the ordinary basics do more than any countdown: steady prenatal visits, movement your provider approves, and good hydration — needs rise in pregnancy, and our water intake calculator gives a sensible baseline to build on. This guide is educational and is not medical advice; your due date, your dating scan, and anything about your pregnancy should be confirmed and interpreted with your own healthcare provider.

Sources: American College of Obstetricians and Gynecologists, Committee Opinion No. 700, "Methods for Estimating the Due Date," Obstetrics & Gynecology (2017); American College of Obstetricians and Gynecologists, Committee Opinion No. 579, "Definition of Term Pregnancy," Obstetrics & Gynecology (2013); Jukic AM et al., "Length of human pregnancy and contributors to its natural variation," Human Reproduction (2013); Mittendorf R et al., "The Length of Uncomplicated Human Gestation," Obstetrics & Gynecology (1990); Lawson GW, "Naegele's rule and the length of pregnancy – a review," Australian and New Zealand Journal of Obstetrics and Gynaecology (2021); Centers for Disease Control and Prevention, National Center for Health Statistics, "Shifts in the Distribution of Births by Gestational Age, United States, 2014–2022," National Vital Statistics Reports 73(1) (2024); Pregnancy Dating, StatPearls, StatPearls Publishing / NCBI Bookshelf (2022).