How accurate is a pregnancy due date? LMP vs dating scan explained

A pregnancy due date looks precise.

14 April.

One exact day.

But biologically, it is not an appointment.

A due date is an estimated due date (EDD) used to place the pregnancy on a gestational timeline. It helps clinicians interpret growth, plan tests and decide when care is due.

It is not a prediction that labour will begin on that specific date.

ACOG notes that only about 1 in 20 women give birth on their estimated due date.

That is why the useful question is not:

Will the baby arrive on my due date?

It is:

How accurately has the pregnancy been dated, and what information was used to assign the EDD?

The Pregnancy Due Date & Timeline Calculator can show the standard calendar calculation, but a clinician-assigned date from your maternity team should take priority over a website estimate.

For the Irish antenatal calendar itself, see Pregnancy week by week in Ireland: GP, hospital and scan schedule.

The two common starting points

Pregnancy dating usually begins from one of two pieces of information:

1. the first day of the last menstrual period (LMP);
2. an ultrasound dating measurement, particularly in the first trimester.

They can produce slightly different dates.

That does not necessarily mean anything is wrong.

They are estimating gestational age using different information.

How the LMP due date is calculated

The conventional LMP method dates pregnancy from:

the first day of the last menstrual period

and adds:

280 days — 40 weeks.

That is the method used by the HSE’s pregnancy due date calculator.

HSE: pregnancy due date calculator.

The calculation is simple.

The assumptions behind it are less simple.

Why pregnancy is already “two weeks pregnant” around conception

Pregnancy is conventionally dated from the LMP rather than from fertilisation.

In the classic 28-day-cycle model:

• day 1 = first day of the period;
• ovulation is assumed around day 14;
• conception occurs around that time;
• gestational age is already roughly two weeks at conception.

So a pregnancy described as:

10 weeks gestation

does not normally mean conception happened exactly 10 weeks ago.

The gestational clock began at the LMP.

The LMP calculation assumes a 28-day cycle

ACOG notes that the standard 280-day LMP method assumes:

• a regular 28-day menstrual cycle;
• ovulation around day 14.

Real cycles are not all 28 days.

Ovulation does not always occur on day 14.

This is one major reason an LMP date and an ultrasound date can disagree.

Example: ovulation happened later

Imagine the LMP calculation assumes ovulation on day 14.

But ovulation actually occurred on day 21.

That is a difference of:

7 days.

An early embryo may therefore measure about a week “behind” the LMP-based gestational age even when development is entirely consistent with the later ovulation date.

The dating scan can help resolve that discrepancy.

LMP recall can also be uncertain

The first day of a period can be difficult to recall exactly.

Other sources of uncertainty include:

• irregular cycles;
• recent hormonal contraception;
• breastfeeding;
• bleeding that was mistaken for a normal period;
• very long or short cycles;
• uncertain dates.

If the LMP is unreliable, ultrasound dating becomes particularly useful.

What does a dating scan do in Ireland?

HSE says all pregnant women in Ireland are offered a dating scan, which should be done before:

15 weeks.

The scan aims to assess:

• fetal heartbeat;
• expected birth date;
• fetal size;
• whether there is more than one baby.

HSE: dating scan.

In early pregnancy, the sonographer can use measurements of the embryo or fetus to estimate gestational age.

What is crown-rump length?

In the first trimester, the key dating measurement is usually the:

crown-rump length — CRL

This measures from the top of the fetal head to the bottom of the torso.

ACOG, AIUM and SMFM state that ultrasound measurement in the first trimester — up to and including 13 weeks + 6 days — is the most accurate method to establish or confirm gestational age.

CRL-based dating in this period is typically accurate to about:

±5–7 days.

ACOG: Methods for Estimating the Due Date.

That still is not perfect to the exact day.

It is simply more accurate than later ultrasound dating.

Why early ultrasound is more accurate for dating

Early in pregnancy, normal embryos and fetuses have relatively similar growth patterns.

As pregnancy progresses, natural differences in fetal size become greater.

That means a measurement later in pregnancy is increasingly a mixture of:

• gestational age;
• individual fetal size.

This is why ultrasound gets less precise for dating as pregnancy advances.

First trimester: approximately ±5–7 days

ACOG gives first-trimester CRL dating an accuracy of approximately:

±5–7 days.

It also notes that CRL measurements are generally more accurate earlier in the first trimester.

This does not mean a fetus measuring six days different from LMP is automatically abnormal.

It describes the precision of the dating method.

Early second trimester: less precise

Between roughly:

14 weeks and 21 weeks + 6 days

ACOG cites ultrasound dating accuracy of approximately:

±7–10 days.

Later in the second trimester the uncertainty becomes wider.

This is one reason an early established due date is usually not repeatedly rewritten based on later growth scans.

Third trimester dating is much less reliable

At 28 weeks and beyond, ACOG describes ultrasound dating as the least reliable period, with accuracy around:

±21–30 days.

At that stage, a fetus may simply be genetically smaller or larger.

Redating a pregnancy from a late scan can therefore mistake a growth difference for an age difference.

Why did my due date change after the scan?

The common reason is that the ultrasound estimate differs enough from the LMP estimate that the clinical team considers the scan the better dating method.

Possible explanations include:

• later or earlier ovulation;
• irregular cycle length;
• uncertain LMP;
• incorrect recall of the LMP;
• early bleeding being mistaken for a period.

A change in due date does not by itself mean that the fetus has suddenly changed its growth rate.

It may simply mean the estimated start point of the pregnancy has been refined.

How big a difference can trigger redating?

Clinical redating is not based on one universal “any difference means change” rule.

ACOG’s guidance gives thresholds that depend on gestational age.

In the first trimester:

• before 9 weeks, a discrepancy of more than about 5 days can support changing the EDD;
• from 9 weeks to 13 weeks + 6 days, a discrepancy of more than about 7 days can support changing the EDD.

Smaller discrepancies can be interpreted in the context of:

• how certain the LMP is;
• how early the scan was performed;
• the clinical picture.

These are clinical guidelines, not instructions for a patient to redetermine their own date at home.

Example: LMP says 9 weeks, scan says 8 weeks + 2 days

Difference:

5 days.

That may or may not lead the clinical team to change the EDD depending on the exact gestational age, scan quality and confidence in the LMP.

The important point is that:

one small difference does not automatically mean the due date is wrong.

Example: LMP and scan differ by 10 days

A 10-day difference in the first trimester is more likely to be clinically meaningful for dating.

If the LMP is uncertain or cycles are irregular, the early ultrasound may become the main basis for the assigned EDD.

Once the clinical team establishes the pregnancy date, later scans are usually interpreted relative to that date.

A later scan should not normally keep moving the due date

This is an important point.

ACOG recommends that once the EDD has been established from LMP and/or the first accurate ultrasound, subsequent changes should be rare.

If a later scan shows the fetus is measuring ahead or behind, that may be a growth question rather than a dating question.

This distinction matters because repeatedly changing the date could conceal abnormal growth.

Dating and growth are different questions

A scan can ask:

Dating question

How far along is this pregnancy likely to be?

Growth question

Is the fetus growing as expected for the already established gestational age?

Early ultrasound is particularly good for the first question.

Later ultrasound is commonly used for the second.

Conflating them can create unnecessary anxiety.

Why the due date matters clinically

A due date is not just a countdown.

Gestational age influences the timing and interpretation of:

• antenatal appointments;
• screening;
• fetal anatomy assessment;
• viability discussions;
• growth monitoring;
• preterm-birth definitions;
• post-term management.

That is why getting the best obstetric estimate early matters even though the baby probably will not be born on the exact EDD.

The Irish anatomy scan comes later

HSE says the fetal anatomy scan is usually performed at:

18–22 weeks.

Its purpose is primarily to assess fetal development and anatomy.

HSE: fetal anatomy scan.

A national HSE clinical guideline says the fetal anatomy scan should only be used to confirm the estimated due date if a first-trimester ultrasound was not performed.

So an established early due date is generally not supposed to bounce around with every later scan.

What if my cycles are longer than 28 days?

A standard LMP calculator does not know your actual ovulation date.

For example, someone with a consistent 35-day cycle may ovulate later than the day-14 assumption built into the classic calculation.

That can produce an LMP estimate that looks earlier than the scan estimate.

A dating scan is therefore valuable when cycle timing differs from the textbook model.

What if my cycles are irregular?

The uncertainty is larger.

If cycle length varies substantially, LMP tells us when bleeding began but not reliably when ovulation occurred.

An early ultrasound measurement can provide a better estimate of gestational age.

Your maternity team decides how to assign the clinical date.

What if I know the exact conception date?

Even then, pregnancy is still documented using gestational age, not “time since fertilisation.”

For spontaneous conception, the exact day of fertilisation is often impossible to know.

Sex can occur days before ovulation because sperm can survive in the reproductive tract.

Ovulation timing itself may not be known precisely.

So an estimated conception date on a website is not the same thing as a clinically established date.

IVF is different

Pregnancies resulting from assisted reproductive technology can be dated from known treatment timing.

ACOG recommends using the ART-derived gestational age, based on the age of the embryo and the date of transfer, to assign the EDD.

This is more precise than trying to back-calculate from an LMP that may not be relevant to the treatment cycle.

The Compound calculator does not attempt to reconstruct IVF dating from transfer details.

Use the due date assigned by the fertility or maternity team.

What does the calculator actually calculate?

The Pregnancy Due Date & Timeline Calculator accepts either:

Last menstrual period

If no clinical due date has been assigned, it adds:

280 days

to the first day of the LMP.

Clinician-assigned due date

If you enter a due date already assigned by your maternity team or dating scan, that date takes priority.

The tool then displays:

• estimated / assigned due date;
• gestational age today;
• trimester;
• approximate conception date from the dating convention;
• 12-week point;
• anatomy-scan window;
• 37-week point;
• 42-week point.

It is a timeline tool, not a diagnostic tool.

The conception date shown by the calculator is deliberately labelled approximate

The calculator subtracts:

266 days

from the due date.

That corresponds to the classic model of conception around two weeks after LMP.

It does not mean fertilisation definitely occurred on that calendar date.

Ovulation varies.

Fertilisation timing varies.

The field is for orientation only.

Why the 12-week point is not necessarily your appointment date

The calculator shows the calendar date on which the pregnancy reaches 12 weeks.

Your actual:

• booking appointment;
• dating scan;
• blood tests;
• screening;
• follow-up

may occur on a different date.

Hospital schedules and individual clinical circumstances vary.

Follow the appointments issued by your maternity unit.

Why gestational age may look strange immediately after the due date changes

Imagine your LMP-based date initially placed you at:

10 weeks + 4 days

but the dating scan reassigns the pregnancy to:

9 weeks + 5 days.

The pregnancy has not moved backward in time.

The clinical estimate of where gestation began has changed.

From that point onward, the revised date is used to calculate gestational age.

Does a smaller CRL mean poor growth?

Not necessarily.

A single early measurement that gives a different gestational age may simply indicate the pregnancy was conceived later than the LMP model assumed.

Assessment of viability or abnormal growth requires the clinical context, including:

• scan findings;
• heartbeat where expected;
• repeat measurements where appropriate;
• symptoms;
• gestational age.

A website should not infer viability from CRL alone.

Does a larger CRL mean the baby will be large?

Not in any simple way.

Early CRL is primarily used for dating.

Birth weight depends on many later factors.

A fetus measuring a few days ahead in the first trimester does not mean you can extrapolate that difference linearly to birth size.

What if the scan and LMP agree closely?

That is common and reassuring from a dating perspective.

If the early scan is consistent with the LMP date, the EDD may simply remain unchanged.

The scan still provides other information, including:

• confirming location and viability where visualised;
• number of fetuses;
• fetal size;
• expected birth date.

What if the scan date keeps changing on private scans?

A printed gestational-age estimate on an ultrasound machine is not necessarily the same thing as your officially assigned EDD.

Different measurements can produce slightly different age estimates.

If you have an established clinical due date, do not assume every later machine-generated estimate replaces it.

Use the date in your maternity record unless your clinical team formally changes it.

Private scans do not replace maternity care

HSE notes that people may choose private ultrasound scans for reassurance, earlier imaging, fetal sex or additional images.

But if a concern is identified, follow-up may need to occur through the maternity hospital.

The due date used for clinical care should come from your maternity record rather than from repeatedly comparing private scan printouts.

Only around 5% give birth on the exact due date

This statistic helps put the whole topic in perspective.

ACOG states that only about:

1 in 20

women give birth on their due dates.

So even a very well-dated pregnancy does not produce a precise delivery-day prediction.

Labour is biological, not scheduled by the EDD.

“Due date” is therefore slightly misleading language

A more accurate mental model is:

estimated centre point of a clinically useful pregnancy timeline.

That date helps define:

• 37 weeks;
• 40 weeks;
• 41 weeks;
• post-term timing.

But it should not create the expectation that the baby is late at 00:01 the morning after the EDD.

Your maternity team will advise you on monitoring and management as pregnancy progresses.

The practical hierarchy

If you are trying to work out which date to use:

1. Use the due date in your maternity record

That is the clinically assigned EDD.

2. If no date has been assigned yet, LMP gives a reasonable first estimate

The HSE calculator uses the standard 40-week convention.

3. Early ultrasound can refine that estimate

First-trimester CRL is the most accurate ultrasound dating method.

4. Do not keep redating from later growth scans yourself

Later measurements are less precise for dating and increasingly reflect individual growth.

5. Ask the maternity team if two documents show different dates

They can tell you which EDD is being used for clinical care.

When the calculator is useful

The Pregnancy Due Date & Timeline Calculator is useful when you want to:

• estimate an initial due date from LMP;
• see approximate gestational age today;
• map calendar milestones;
• use an already assigned due date as the timeline anchor.

It is not useful for deciding whether:

• a pregnancy is viable;
• fetal growth is normal;
• a scan result is concerning;
• a due date should be clinically changed;
• symptoms need assessment.

Those questions belong with the maternity team.

The practical conclusion

Pregnancy due dates are useful because pregnancy care needs a consistent clock.

But the clock is an estimate.

The standard LMP method assumes:

40 weeks from the first day of the last period

and a textbook cycle pattern.

An early dating scan can refine that estimate because first-trimester crown-rump length is a more accurate method of establishing gestational age.

HSE offers dating scans before:

15 weeks.

International obstetric guidance considers first-trimester ultrasound through 13 weeks + 6 days the most accurate period for ultrasound dating, with typical precision around:

±5–7 days.

Once the maternity team has assigned the due date, that clinical date is the one to use.

And even with excellent dating, the EDD remains exactly what its name says:

estimated.