How Is Your Due Date Calculated — And How Accurate Is It?

The first thing most people want after a positive test is a date. And they get one — precise, specific, printed on paperwork, immediately shared with family.

What almost nobody explains is that the date is an estimate built on assumptions that don’t hold for everyone, and that roughly 96% of babies arrive on a different day. Understanding how the number is produced makes the whole thing far less stressful.

The formula

The standard method is Naegele’s rule, and it works from the first day of your last menstrual period — not from conception.

Take the first day of your last period, add 280 days. That’s 40 weeks. An equivalent shortcut: subtract three months and add seven days.

If your last period began on March 1, your estimated due date is December 6.

The Due Date Calculator does this and gives you the corresponding trimester dates and week-by-week milestones.

Why it counts from your period, not conception

This is the part that confuses everyone, and it’s worth getting straight.

Pregnancy is measured in gestational age, counted from the first day of your last period. But conception doesn’t happen then — ovulation is roughly two weeks later.

So at the moment you conceive, you’re already considered “two weeks pregnant.” By the time a test turns positive, you’re typically around four weeks. The first two weeks of your pregnancy, by this counting, happened before there was a pregnancy.

It seems absurd until you see the reason: the date of your last period is something you can usually pin down. The date of conception almost never is. Sperm can survive several days in the reproductive tract, so even a woman who knows exactly when she had intercourse doesn’t know exactly when fertilisation occurred. Clinicians use the measurable landmark rather than the unknowable one.

Two assumptions baked into the formula

Naegele’s rule assumes a 28-day cycle with ovulation on day 14. Both are averages, and plenty of people don’t match them.

If your cycles run longer — say 35 days — you ovulate later, which means you conceived later, which means the standard formula puts your due date about a week too early. Shorter cycles push it the other way.

The adjustment is straightforward: add the difference between your cycle length and 28 days. On a 35-day cycle, add seven days.

If your cycles are irregular, or you don’t know when your last period started, or you conceived while on hormonal contraception, the last-period method may not give you anything reliable at all. This is where the calculator’s answer should be treated as a placeholder until a scan replaces it.

Why ultrasound dating usually wins

An early ultrasound measures the embryo directly, and in the first trimester that measurement is the most accurate dating tool available.

Between about 8 and 13 weeks, embryos grow at a remarkably consistent rate, so crown-rump length predicts gestational age to within about five to seven days. Later scans get progressively less reliable, because by the third trimester babies differ a great deal in size for reasons that have nothing to do with age.

Standard practice: if an early scan disagrees with the last-period calculation by more than about a week, the scan date is adopted. This is why some people have their due date changed at a first appointment — the formula has been superseded by a measurement, which is a straightforward upgrade rather than anything to worry about.

What “due date” actually means

Here’s the fact that changes how the whole thing feels.

Only about 4% of babies are born on their estimated due date. Around 80% arrive somewhere between 37 and 42 weeks. First pregnancies tend to run slightly longer than subsequent ones.

The medical terminology reflects this. A pregnancy is now described as:

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

Note that 40 weeks — the due date itself — sits in the middle of full term, not at the end of it. It isn’t a deadline. It’s the centre point of a normal five-week window.

A more useful mental model is a due month rather than a due date. Somewhere in that window, most likely. Which is also why “any day now” messages from relatives starting at week 38 are, statistically speaking, about two weeks premature.

What the date is actually for

The estimate isn’t really for planning your leave. It’s a clinical reference point that determines:

when screening tests are scheduled, several of which are only valid in narrow windows
how growth is assessed against expected measurements
when a pregnancy is considered post-term, and monitoring or induction gets discussed
what interventions are appropriate if labour starts early

Accurate dating matters most for those decisions. Whether the baby arrives on the 6th or the 14th matters far less than whether the care team knows how far along you are when something needs assessing.

Practical takeaway

Use the calculator to get an initial estimate. Adjust it if your cycle isn’t 28 days. Expect the number to be refined at your first scan, and don’t be surprised or alarmed when it moves.

Then treat it loosely. Have things ready by 37 weeks and be prepared for anything up to 42. The date on the paperwork is a well-informed estimate, not an appointment.

This article is general information and not medical advice. If you are pregnant or think you may be, speak with a doctor or midwife, who can date the pregnancy properly and advise on your specific circumstances.