How Accurate Is a Pregnancy Due Date?

What “accurate” means for a due date

People use “accurate” in two different ways when they talk about due dates, and it helps to separate them:

• Is the pregnancy dated correctly? This is about how well the due date reflects how far along the pregnancy really is. Dating can be improved: ultrasound measurement up to and including 13 weeks 6 days is the most accurate method to establish or confirm how far along a pregnancy is.³
• Will the baby arrive on that day? This is about predicting the birth itself. Here a single date can never be precise, because pregnancy normally lasts anywhere from 37 to 42 weeks.²

A due date can be well dated and still not be the day of birth. That isn’t a flaw in the calculation — it’s what an estimated date is.

How often are babies born on their due date?

Not often. In one Australian study of 18,708 women whose labour started spontaneously, only 5% of births occurred on the estimated date of birth, regardless of the dating method used.¹ In the same study, approximately 66% of births occurred within 7 days of the estimated date.¹

These are findings from one group of pregnancies. They describe what happened across that study, not what will happen in any one pregnancy.

The last row is simply the remainder: if about two-thirds of births fell within 7 days, about a third fell further away.

What the study figures do — and don’t — tell you

The figures above come from 18,708 Australian women with spontaneous labour and singleton births between 2007 and 2011.¹ A few things follow from that:

• 5% is roughly 1 in 20. In that group, about one birth in twenty fell on the estimated date itself.
• The dating method didn’t change the 5%. The share born on the estimated date was the same regardless of how the pregnancy had been dated.¹
• It describes one population. The study looked at births where labour started on its own. It describes what happened across that group and can’t tell you the chance for your own pregnancy.

What “within a week” looks like on a calendar

It can help to turn your due date into a window. Take an example due date of 15 April 2027:

• 7 days before: 8 April 2027
• Estimated due date: 15 April 2027
• 7 days after: 22 April 2027

In the study above, about two-thirds of births fell inside a window like this one — and only 5% on the middle date itself.¹ You can calculate your estimated due date to see your own dates and the term categories around them.

Why a due date can’t be exact

The calculation makes assumptions

The standard due date is 280 days after the first day of the last period.³ That convention assumes a regular 28-day cycle with ovulation on day 14, and it doesn’t account for irregular cycles, variable ovulation or an inaccurately remembered date.³ How the number is worked out is explained in how a due date is calculated.

Pregnancy length varies naturally

Even when the starting point is known precisely, pregnancies differ in length. In a study that identified ovulation with hormone tests, the time from ovulation to birth had a median of 268 days, and pregnancy length varied over a 37-day range even after excluding preterm births.⁴ No calculation can tell where in that range a particular pregnancy will fall.

The normal range: 37 to 42 weeks

The NHS says pregnancy normally lasts from 37 to 42 weeks from the first day of the last period.² The WHO defines term as 37 to less than 42 completed weeks (259 to 293 days), with pre-term below 37 completed weeks and post-term from 42 completed weeks.⁵ ACOG divides term into narrower categories:⁶

A baby born before 37 completed weeks of pregnancy is described as preterm.⁷ Your due date (40 weeks 0 days) sits inside the full-term band, not at its end.

Born a few days early or late: was the due date wrong?

Not necessarily. The due date marks the day a pregnancy reaches 40 weeks 0 days. A birth at, say, 38 weeks or 41 weeks falls within the range in which pregnancy normally ends,² and falls in ACOG’s early-term or late-term category.⁶

Two things can make a birth date differ from the due date, and they’re easy to confuse:

• The dating itself was off — for example, because ovulation didn’t happen around day 14 or the last period date was misremembered.³ This is what an early scan is used to check.
• The pregnancy was simply shorter or longer than 40 weeks. Pregnancy length varies naturally, over a 37-day range in one study even after excluding preterm births.⁴ A correctly dated pregnancy can still end days before or after its due date.

So a baby arriving a week “early” or “late” isn’t, on its own, a sign that the calculation was wrong.

Is a due date from your last period less accurate than a scan?

For dating the pregnancy, yes: ultrasound measurement up to and including 13 weeks 6 days is the most accurate method to establish or confirm how far along a pregnancy is.³ That’s why an early scan can lead to a changed due date when it differs enough from the period-based date.³

But better dating doesn’t make the birth date predictable. In the study above, only 5% of births fell on the estimated date whichever dating method was used.¹ Read more about ultrasound vs LMP dating.

Can your due date be wrong?

A due date worked out from your last period can turn out to be off — for example, if ovulation happened later than day 14 or the date was misremembered.³ That is exactly what an early scan is used to check. Once the due date has been set from the LMP and/or the first accurate ultrasound, later changes should be reserved for rare circumstances.³

When ultrasound dating matters most

A due date from your last period rests on assumptions, so a scan is especially useful when those assumptions may not hold:

• You aren’t sure of your last period date. If you don’t know or aren’t sure of it, speak to a midwife or GP.²
• Your cycles are irregular. The standard calculation assumes a regular 28-day cycle.³ See due dates with irregular periods.
• The scan and your period disagree. Guidance changes the due date only when the difference is larger than a threshold for that stage of pregnancy.³

With IVF, the due date comes from the embryo transfer date rather than from a scan or your period.³

Reading your calculator result

When you use the due date calculator, each part of the result answers a slightly different question:

If the result differs from a due date your healthcare team has given you, use theirs.

How to use your due date

• Treat it as one point within a range, not a deadline. The term categories above show the dates around it.
• Use the date your healthcare team gives you if it differs from a calculator’s, especially after a scan.
• Count weeks from it. Your due date also tells you how far along you are; you can find out how many weeks pregnant you are from it at any time.

Your midwife or doctor can talk you through what your dates mean for your care.

Frequently asked questions

What percentage of babies are born on their due date?

In one Australian study of 18,708 spontaneous-labour births, 5%.¹ That figure comes from one study and isn’t a prediction for any individual pregnancy.

Is a due date 39 or 40 weeks?

The estimated due date is the day you reach 40 weeks 0 days. ACOG’s full-term category runs from 39 weeks 0 days to 40 weeks 6 days.⁶

How many babies arrive within a week of the due date?

In the same study, approximately 66% of births occurred within 7 days of the estimated date.¹

If my baby is born at 38 weeks, was my due date wrong?

Not necessarily. 38 weeks falls in ACOG’s early-term category (37 weeks 0 days to 38 weeks 6 days),⁶ within the range in which pregnancy normally ends.

Does an early scan make the birth date more predictable?

An early scan improves the dating of the pregnancy, but in the study above only 5% of births fell on the estimated date whichever dating method was used.¹

What does EDD mean?

Estimated due date — the “estimated” matters. Read more about what EDD means.

Medical statements in this article are linked to the numbered sources listed below: published clinical guidance and peer-reviewed research. It has not been reviewed by a named clinician. Read how content is written in our editorial policy, and see our medical disclaimer.