pregnancy-birth

How Your Due Date Is Calculated — and What It Actually Means

By Dr. Adeeb Alshahrour · July 16, 2023

Your estimated due date is the most quoted number in your pregnancy and the least well understood. It is not a prediction of the day you will give birth. It is a reference point — and it is a load-bearing one, because almost every test window in pregnancy is measured from it.

Get the dating right, and the nuchal translucency scan, the cell-free DNA test, the anatomy scan, the glucose test, the group B strep swab, and any decision about induction all land where they should. Get it wrong by a week and several of those windows are missed entirely.

Where 40 weeks comes from

The estimated due date, or EDD, is 280 days from the first day of your last menstrual period — 40 weeks.

That formula is Naegele's rule, and the arithmetic version is: take the first day of your last period, add one year, subtract three months, add seven days. A last period starting 1 March gives a due date of 8 December.

The odd part is that pregnancy is dated from a day on which you were not pregnant. Conception typically happens about two weeks after the start of a period, so a "10-week" pregnancy contains an embryo that is roughly 8 weeks old. Everyone in obstetrics uses this convention, so it is consistent — but it is why the numbers may not match your own arithmetic about when you conceived.

Naegele's rule also assumes a 28-day cycle with ovulation on day 14. Plenty of people do not have that cycle. If yours is 35 days, you ovulate around a week later, and the LMP-derived due date will be about a week too early. This is one of the main reasons LMP dating goes wrong.

Dating from the LMP is also unreliable if you have irregular cycles, PCOS, recently stopped hormonal contraception, are breastfeeding, had bleeding early in pregnancy that was mistaken for a period, or simply do not remember the date precisely. That last one is far more common than people admit, and it is nothing to be embarrassed about.

Why the first-trimester ultrasound is more accurate

In the first trimester, all embryos grow at almost exactly the same rate. Genetics, nutrition, and everything else that later makes babies different sizes has not yet had time to matter. That biological uniformity is what makes early ultrasound dating so good.

The measurement is the crown-rump length (CRL) — the length of the embryo from the top of the head to the bottom of the torso. Measured before 13+6 weeks, it estimates gestational age to within about 5 to 7 days.

Accuracy falls off steadily after that, because babies genuinely do start growing at different rates. A scan at 20 weeks is accurate to roughly ±10 days; at 30 weeks, to about ±3 weeks. A third-trimester scan is a poor way to date a pregnancy — a big baby at 32 weeks looks the same on ultrasound as an average baby further along. This is worth knowing, because a late scan that suggests different dates is usually telling you about growth, not about timing.

When the scan date replaces your LMP date

Both dates are compared, and the ultrasound only overrides the LMP if the difference is larger than expected measurement error. ACOG's thresholds:

Before 9+0 weeks: change the due date if the scan differs by more than 5 days
9+0 to 15+6 weeks: change it if the difference is more than 7 days
16+0 to 21+6 weeks: change it if the difference is more than 10 days
22+0 to 27+6 weeks: change it if the difference is more than 14 days
28+0 weeks and beyond: change it if the difference is more than 21 days, and treat the result with caution

If the difference is smaller than the threshold, the LMP date stands. The logic is that within measurement error, a well-remembered period date is as good as a scan and there is no reason to move a number the whole pregnancy depends on.

Two rules follow from this:

Your due date is set once, as early as possible, and then it does not move. Once the EDD is established it should rarely be changed. Later scans measure growth against that date; they do not renegotiate it. If a scan at 32 weeks measures ahead, the baby is large — the due date does not shift.
A pregnancy conceived through IVF is dated from the embryo, not from a period or a scan. The age of the embryo and the transfer date give an exact conception date, so the due date is known precisely and ultrasound does not override it.

What "due date" actually means

Only about 4 percent of babies are born on their estimated due date. That is the number worth carrying around. Spontaneous labour is spread across several weeks either side, and this is normal biology rather than something going wrong.

The vocabulary that has replaced "due date" in clinical use is more informative:

Early term: 37+0 to 38+6 weeks
Full term: 39+0 to 40+6 weeks
Late term: 41+0 to 41+6 weeks
Post-term: 42+0 weeks and beyond

These categories exist because outcomes differ measurably between them. Babies born at 39 weeks do better than babies born at 37, which is why an elective delivery without a medical reason is scheduled at 39 weeks and not earlier. At the other end, risk begins to rise again after 41 weeks, which is why a conversation about induction usually happens between 41+0 and 42+0 weeks with increased monitoring in the interim.

None of this arithmetic works if the dating is wrong. A due date that is a week late by mistake means an "induction at 41 weeks" that is really at 40 — and a due date that is a week early means what looks like 41 weeks is actually 42.

The scan windows your due date sets

Dating scan, 6 to 12 weeks

Confirms the pregnancy is in the uterus, how many embryos there are, whether there is a heartbeat, and the CRL. Often transvaginal at this stage, which gives a much clearer picture and is safe. If you have had bleeding or pain, this scan happens sooner — see bleeding in pregnancy.

Nuchal translucency scan, 11+0 to 13+6 weeks

The correct window is 11+0 to 13+6 weeks, corresponding to a CRL of 45 to 84 mm. Not "11 weeks and 6 days" — a scan at 11+2 is squarely inside the window and perfectly valid.

This measurement of the fluid at the back of the baby's neck is part of first-trimester screening for chromosomal conditions and is usually combined with blood markers or cell-free DNA. The window is fixed by biology: before 11+0 the structure cannot be measured reliably, and after 13+6 the fluid resorbs and the number is no longer interpretable. Miss it and it cannot be repeated later.

Within that window, 12+0 to 13+0 weeks tends to give the clearest views of the fetal heart, bladder, and other early structures if you also want a good look at anatomy. That is a preference, not a requirement.

Cell-free DNA screening (NIPT) can be drawn from 10 weeks onward and is offered to every patient regardless of age.

Anatomy scan, 18 to 22 weeks

The detailed head-to-toe survey: brain, face, spine, heart, kidneys, stomach, abdominal wall, limbs, plus placental position, cord, and amniotic fluid volume. The standard window is 18 to 22 weeks, and many units aim for 19 to 21 weeks as the sweet spot — early enough to leave time for follow-up if something needs a closer look, late enough that the structures are large enough to see well.

If the baby is lying awkwardly, or if visibility is limited, a repeat scan is arranged. That is routine and not a sign of a problem. More on how the imaging itself works is in our guide to ultrasound.

Later scans

Growth scans, cervical length measurement, and biophysical profiles are arranged when there is a reason — a growth concern, diabetes, high blood pressure, twins, reduced movement, or a low-lying placenta seen earlier. Their timing is driven by the clinical question, not by the calendar.

Practical points

Bring the date of your last period to your first visit if you have it. If you do not, say so plainly rather than guessing — a wrong date is worse than no date.
Do not calculate your own dates from an online tool and act on them. Online calculators use Naegele's rule and know nothing about your cycle length or your scan.
Ask what your EDD is based on — LMP, ultrasound, or IVF transfer. It is written in your chart and you are entitled to know.
If different providers quote you different due dates, flag it. It usually means someone is working from the LMP and someone else from the scan, and it needs to be resolved once rather than repeatedly.

Everything above is why the first prenatal visit happens early. See prenatal care for what that visit involves, and our complete guide to prenatal care for the full schedule of visits and tests that your due date anchors.

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Medical disclaimer

This article is general health information, not medical advice for any individual. Your estimated due date and scan timing must be set by the clinician caring for you. Contact your obstetrician the same day for vaginal bleeding, leaking fluid, severe headache or visual changes, or decreased fetal movement. In an emergency, call 911.