Ultrasound Dating vs LMP: Which Due Date Is More Accurate?
Your estimated due date (EDD) is rarely a single fixed number that stays constant throughout pregnancy. Most women find that their due date is adjusted — sometimes more than once — as new information becomes available. Understanding why this happens, which measurement method is more accurate at each stage, and what the guidelines actually say helps you make sense of these revisions rather than finding them unsettling.
The Two Starting Points: LMP and Ultrasound
LMP-based dating
The last menstrual period method is the first calculation used in almost all pregnancies. Adding 280 days (40 weeks) to the first day of your last period gives an initial EDD — this is Naegele's Rule, which has been in clinical use for nearly 200 years.
The limitation is that LMP-based dating assumes a regular 28-day cycle with ovulation on day 14. For women with irregular cycles, cycles significantly longer or shorter than 28 days, or those who cannot recall their LMP date accurately, the resulting EDD may be off by a week or more.
Ultrasound-based dating
Early ultrasound measures the fetus directly, removing the cycle-length assumption. The most accurate measurement for dating purposes is the crown-rump length (CRL) — measured from the top of the head to the bottom of the spine. This measurement is taken during the first trimester, ideally between 10 and 13 weeks.
At this stage of development, all fetuses of the same gestational age are very similar in size. There is minimal biological variation in CRL at 10–13 weeks, which is why first-trimester ultrasound is significantly more accurate than LMP for establishing gestational age.
Accuracy Comparison: Scan vs LMP
| Method | Optimal Window | Accuracy (margin of error) | Notes |
|---|---|---|---|
| LMP (regular cycle) | All trimesters | ± 1–2 weeks | Less accurate with irregular cycles or uncertain LMP |
| First trimester scan (CRL) | 10–13 weeks | ± 5–7 days | Most accurate single method for dating |
| Second trimester scan | 14–20 weeks | ± 7–14 days | Less reliable for dating; used for anatomy review |
| Third trimester scan | 28+ weeks | ± 2–3 weeks | Not used for dating; assesses growth and wellbeing |
The earlier the ultrasound is performed, the more accurate it is for dating purposes. This is why the first-trimester dating scan is the gold standard in countries where it is routinely offered.
When Scan Results Override the LMP Date
Different professional guidelines set specific thresholds for when an ultrasound finding should replace the LMP-based date. The key guidelines are:
ACOG (American College of Obstetricians and Gynecologists)
ACOG recommends revising the EDD based on ultrasound when the discrepancy between scan and LMP exceeds:
| Gestational Age at Scan | Revise EDD if Discrepancy Exceeds |
|---|---|
| Under 9 weeks | 5 days |
| 9 weeks 0 days to 15 weeks 6 days | 7 days |
| 16 weeks to 21 weeks 6 days | 10 days |
| 22 weeks or later | Not recommended to revise based on scan alone |
In practice, the majority of women who have a first-trimester scan will have their EDD revised to the scan-based date if any discrepancy exists beyond these thresholds.
NHS (National Health Service, England)
The NHS approach differs slightly. Rather than applying discrepancy thresholds, NHS guidance uses the dating scan as the primary dating method as a matter of routine — the scan date replaces the LMP date in all cases unless the scan is not technically possible. The LMP is recorded but the scan-based date is what is entered into the maternity records and used for all subsequent care planning, including induction timing.
RCOG (Royal College of Obstetricians and Gynaecologists)
The RCOG supports the NHS approach and recommends that all women are offered an ultrasound scan at 10–14 weeks. Crown-rump length is the preferred measurement for establishing gestational age, with head circumference used when the CRL exceeds 84 mm (approximately 14 weeks).
When the LMP Date Is Kept
In some situations, the LMP-based date may be retained or given equal weight:
- Scan and LMP agree closely: If the difference between scan and LMP dates falls within the threshold for the gestational age (for example, within 5 days at under 9 weeks), many providers keep the LMP-derived date unchanged, as the scan has confirmed rather than revised it.
- Late booking: Women who do not have a first-trimester scan — either because they booked late or because the scan was unavailable — will retain the LMP-based date, as no more accurate measurement exists.
- IVF and assisted conception: As noted in our LMP vs Conception Date guide, pregnancies achieved through IVF use the clinic-derived conception date from the outset, and neither LMP nor early scan typically changes this.
How Much Can a Due Date Change?
Revisions at the dating scan are typically modest for women with regular cycles and accurate LMP recall — often just a few days. Revisions of one to two weeks are more common when:
- Cycles are irregular (longer or shorter than 28 days)
- The LMP is recalled imprecisely
- Conception occurred during a month of unusual cycle timing (illness, travel, stress)
A revision of up to two weeks at the dating scan is considered within normal parameters. It does not mean anything has changed with the pregnancy — only that the first calculation used different assumptions than what the scan measurement reveals.
What the Dating Scan Appointment Involves
In the UK (11–14 week scan)
The NHS offers a combined dating and nuchal translucency (NT) scan between 11 weeks and 13 weeks 6 days. This is usually the first ultrasound for most UK pregnancies. At the appointment:
- A sonographer measures the CRL to establish or confirm gestational age
- The NT measurement is taken to assess Down's syndrome risk (as part of combined screening)
- The sonographer checks for a heartbeat and basic structural development
- The estimated due date is established or revised based on CRL
The appointment typically takes 30–45 minutes. A full bladder may be requested for transabdominal scanning, though a transvaginal scan may be used if the image quality is insufficient.
In the USA (10–14 week dating scan)
Practice varies more widely in the USA. Some providers offer a dedicated first-trimester dating scan at 10–14 weeks; others may perform the first scan at 6–8 weeks for viability confirmation, then again at 11–14 weeks. ACOG recommends offering a first-trimester scan to all patients for accurate dating and nuchal translucency screening. The measurements taken and the process are equivalent to the UK approach.
Why the 20-Week Scan Does Not Usually Change Your Due Date
The anatomy scan at 18–22 weeks (called the anomaly scan in the UK or the anatomy survey in the USA) assesses fetal anatomy — the brain, heart, spine, kidneys, limbs, and facial features. It is not a dating scan.
By 20 weeks, fetal growth begins to diverge between individuals. Small babies and large babies are genuinely growing at different rates — this is not measurement error. If a 20-week scan were used to revise gestational age, the revision could incorrectly reclassify a growth-restricted fetus as smaller-for-dates or a larger fetus as further along than it is. This is why routine practice is to fix gestational age at the first-trimester scan and interpret subsequent measurements as growth data relative to that fixed date.
The EDD Is Always an Estimate
It is worth stepping back to note that even the most accurately established EDD is a statistical estimate. Only around 4% of babies are born on their due date. The majority are born within two weeks either side. The due date is clinically useful as an anchor for care planning — for scheduling scans, offering induction at 41–42 weeks, and interpreting growth measurements — but it is not a prediction of the birth day.
Our Pregnancy Due Date Calculator gives an initial EDD based on your LMP and cycle length. Once you have your dating scan result, use the scan-based date for all subsequent planning.
A Practical Timeline: From LMP to Confirmed EDD
Understanding where your EDD comes from at each stage can help you interpret what your midwife or doctor tells you:
| Stage | What Happens | EDD Status |
|---|---|---|
| First appointment (GP/midwife booking) | LMP date recorded; initial EDD calculated | Provisional — based on LMP |
| 10–13 weeks (dating scan) | CRL measured; gestational age established by scan | Official EDD set — replaces LMP if discrepancy exceeds threshold |
| 16 weeks (midwife appointment) | EDD confirmed in maternity notes | Fixed — not normally revised |
| 18–22 weeks (anatomy scan) | Fetal anatomy assessed; growth measurements taken | EDD not revised; growth monitored against fixed date |
| 28–36 weeks (growth scans if indicated) | Fetal size and fluid levels monitored | EDD not revised; growth velocity assessed |
| 40+ weeks | Induction offered; timing based on confirmed EDD | EDD used to determine induction eligibility |
If you received a different EDD from your online calculator than from your dating scan, the scan-based date is the one that matters for your maternity care.
Related Guides
- LMP vs Conception Date: Which Is More Accurate? — how the two calculation methods compare
- Why Due Dates Change During Pregnancy — full explanation of how EDDs are revised
- What Is Gestational Age? — the difference between gestational age and fetal age
- Pregnancy Due Date Calculator — calculate your estimated due date
This guide is for general information only. Dating thresholds are based on ACOG Committee Opinion 700 (2017), NHS Antenatal Care guidance, and RCOG published standards. Always discuss your due date and scan results with your midwife, obstetrician, or GP.