Clinical dating
Ultrasound and due dates: when dating is most accurate
A scan does not watch the moment of conception. It measures development and applies validated dating relationships, with an uncertainty range.
Checked against authoritative sources
Evidence reviewed 6 August 2026 · 9 minute read
General education, not individual medical advice.
The short answer
ACOG says first-trimester ultrasound through 13+6 is the most accurate method to establish or confirm gestational age; crown–rump length dating is accurate to about plus or minus five to seven days.
Why early ultrasound is preferred
Embryonic growth is relatively consistent early in pregnancy, making crown-rump length useful for dating. ACOG identifies first-trimester ultrasound, up to and including 13+6, as the most accurate method to establish or confirm gestational age.
The reported accuracy is still a range of about plus or minus five to seven days. It does not identify an exact conception timestamp.
Sources for this section: American College of Obstetricians and Gynecologists.
How ultrasound and LMP are reconciled
A clinician compares the early ultrasound estimate with the LMP estimate and considers how reliable the LMP date is. ACOG provides discrepancy thresholds that vary with gestational age.
Those thresholds are clinical guidance. A consumer calculator should not ask for a scan measurement and automatically redates the pregnancy.
Sources for this section: American College of Obstetricians and Gynecologists.
When the LMP is uncertain
If the first day of the last period is unknown, bleeding was unusual, or cycles are irregular, LMP dating may be weak. The NHS advises speaking with a midwife or GP when the date is unknown or uncertain.
Bring whatever information you have, but do not force a date into a calculator simply because the field is required.
Sources for this section: NHS, American College of Obstetricians and Gynecologists.
Why later measurements do not keep moving the date
As pregnancy progresses, normal variation in fetal size increases. A later measurement that is larger or smaller than average does not automatically mean the dating was wrong.
ACOG says the EDD should be established and documented when the LMP and first accurate ultrasound are available. Subsequent changes should be rare, discussed, and documented.
Sources for this section: American College of Obstetricians and Gynecologists.
Dating before 22 weeks matters
ACOG uses the clinical term “suboptimally dated” for a pregnancy without an ultrasound that confirms or revises the EDD before 22+0.
This is terminology for care planning, not a label to apply to yourself from an article. Ask the care team how dating is being handled.
Sources for this section: American College of Obstetricians and Gynecologists.
What to enter in a pregnancy app
Before clinical dating, an LMP estimate can provide orientation. After the care team establishes an EDD, update the app to that date and keep it stable.
Do not change it after each scan unless the clinician formally changes the documented EDD.
Sources for this section: American College of Obstetricians and Gynecologists.
Questions that make the dating conversation clearer
Bring the first day of the LMP if known, usual cycle length and variability, relevant bleeding details, and any ART records. Ask which date is being used as the official EDD and whether it was based on LMP, the earliest accurate ultrasound, or ART dating.
If two reports display different dates, ask whether one is a measurement-based estimate and the other is the documented EDD. The distinction matters: a later fetus measurement can differ from average without replacing the established pregnancy date.
Write down the answer rather than relying on a rounded week spoken during the appointment. An exact EDD lets every tracker calculate the same week-plus-day value.
- What is the documented EDD in my record?
- Which source established it?
- Was the LMP considered reliable?
- Should I update the date in my pregnancy tracker?
- Who should I contact if another report shows a different estimate?
Sources for this section: American College of Obstetricians and Gynecologists, NHS.
Common questions
Can a first-trimester ultrasound tell the exact conception date?
No. It estimates gestational age with an uncertainty range; it does not observe fertilization.
Why did my ultrasound date differ from LMP?
LMP recall, cycle length, ovulation timing, and measurement uncertainty can all contribute. Your clinician applies timing-specific guidance.
Should a third-trimester scan change my due date?
Usually an established EDD remains stable. Any change should be clinician-led, discussed, and documented.
Sources and review notes
We favour public-health guidance, clinical bodies, official statistics, and original research. Source links are provided so you can inspect the wording and boundaries yourself.
- 01Methods for Estimating the Due DateAmerican College of Obstetricians and Gynecologists · May 2017; reaffirmed 2025
- 02Pregnancy due date calculatorNHS