OpenMFM · Clinical education
Gestational dating for sonographers and providers
Evaluate menstrual history.
Find the earliest reliable scan.
Compare the dating sources.
Record one justified EDD.
Close the loop.
Dr. Chukwuma Onyeije, MD · Atlanta Perinatal Associates
Updated October 21, 2026 · 25 slides + 10 case questions
Learning objectives
Your task at a new visit is to establish where the assigned date came from.
Separate a recalled last menstrual period (LMP) from a reliable menstrual dating history.
Identify the earliest adequate ultrasound or assisted reproductive technology (ART) record.
Use the appropriate gestational-age window and request clinician review when sources conflict.
Document the final estimated due date (EDD), its basis and the communication plan.
Why it matters
Gestational age is an input to decisions across the entire pregnancy.
Screening windows and fetal growth percentiles depend on the assigned age.
Surveillance, corticosteroid decisions and delivery timing depend on the best obstetric estimate.
A date copied forward without its source can carry uncertainty into every later visit.
Ask · Proposed interview workflow
Ask about certainty and cycle behavior before using the date.
What was the first day of your last normal period? How certain are you? What is your usual cycle length and range? Was that bleeding typical?
Ask about irregular cycles, recent hormonal contraception, lactation or amenorrhea, and fertility treatment. Record unknown information as unknown.
Example: “LMP recalled with certainty; cycles vary from 35–60 days.” The recalled date is certain; the ovulation assumption is unreliable.
Ask · Reliability
LMP-based dating conventionally assumes a 28-day cycle and ovulation around day 14.
Even regular cycles do not prove the exact ovulation date.
Variable cycles or atypical bleeding undermine the assumptions.
Do not use “28 ± 3 days” as a universal pass/fail rule. Document the actual pattern and reconcile with appropriate early imaging.
Retrieve · Proposed records workflow
“Was there any ultrasound before the scan in this referral?”
Emergency department, outside OB office or fertility clinic.
Anatomy scan generates a new machine estimate.
The later estimate appears as the assigned EDD.
Request the original report: scan date, measurements, image adequacy, ultrasound age and the EDD adopted at that time. A patient’s recollection prompts retrieval; it does not substitute for a verified report.
Reconcile · Decision pathway
An outside EDD is a starting point for review.
Use the treatment-derived age; verify embryo age and transfer date.
Check how it was established and retain it when later size differs.
Evaluate LMP reliability and the earliest adequate ultrasound together.
When records conflict or cannot be obtained, document the uncertainty and escalate to the responsible provider. Do not average competing EDDs.
Measure · First trimester
Crown–rump length (CRL) is only as useful as the image used to measure it.
Midsagittal view; neutral posture; sufficient magnification; longest straight crown-to-rump length. Exclude limbs and yolk sac.
Use technically adequate repeated measurements. Reacquire a flexed, extended or off-axis image.
Record image limitations. Do not assign confidence to an inadequate measurement simply because the machine displays a gestational age.
Measure · Composite biometry
BPD + HC + AC + FL: preserve technique before interpreting the estimate.
Thalami and cavum septi pellucidi visible; cerebellum excluded. Usually outer-to-inner skull edges; match the chart convention.
Same axial plane; trace outer bony calvarium. Prefer HC when head shape distorts BPD.
True transverse section at stomach and portal sinus/umbilical vein level; trace the skin line.
Measure the ossified diaphysis; exclude the epiphysis. Avoid foreshortening.
Diagrams are measurement reminders, not anatomical reference images. Use the measurement convention validated for the selected chart.
Interpret · Uncertainty
Approximate ultrasound dating accuracy; the wider band means more uncertainty.
Accuracy ranges are not the thresholds for changing an EDD. Use the next slide’s gestational-age-specific criteria.
Reconcile · Guideline thresholds
Apply ACOG Table 1 only after reviewing LMP reliability and any established earlier dating.
| Gestational age | Measurement | Discrepancy |
|---|---|---|
| ≤8w6d | CRL | >5 days |
| 9w0d–13w6d | CRL | >7 days |
| 14w0d–15w6d | Composite biometry | >7 days |
| 16w0d–21w6d | Composite biometry | >10 days |
| 22w0d–27w6d | Composite biometry | >14 days |
| ≥28w0d | Composite biometry | >21 days* |
> means more than, not equal to. Assess LMP reliability and prior dating first. At 22–27w6d, smaller differences (10–14 days) may justify a change based on timing and LMP reliability.
*If the first ultrasound is in the third trimester, a difference >21 days may support redating. Use the whole clinical picture and follow-up because redating a small fetus can conceal growth restriction.
Reconcile · Protect the anchor
Use the table in the context of the original dating decision.
Review LMP reliability, scan timing, technical quality and available prior studies. Reconcile the evidence into a best obstetric estimate.
Compare current size with the established age. A machine-generated “ultrasound age” is not automatically a replacement EDD.
Subsequent EDD changes should be rare, explained to the patient and clearly documented. A newly retrieved earlier report warrants review of the original decision, not an automatic overwrite.
Case 1 · Conflicting records
Hypothetical referral: the chart lists an anatomy-scan EDD; the patient recalls an 8-week emergency visit.
Obtain the early report and confirm patient, scan date and measurement.
Was it an adequate CRL? What EDD was established? Why did the later report differ?
Provider reconciles the record, documents the final basis and informs the referring team.
Until the discrepancy is resolved, clearly label the working date and outstanding evidence. Avoid silent changes in the ultrasound system.
Case 2 · Uncertain LMP
Hypothetical case: 12w0d by uncertain LMP; technically adequate CRL corresponds to 10w3d.
11 days
84 days − 73 days = 11 days.
Use the earliest reliable ultrasound estimate when LMP is uncertain. First check for an earlier scan or ART record.
Explain: “Your period date is uncertain. We are using the early ultrasound to establish the best estimate and recording its source.”
Case 3 · Arithmetic and boundaries
Weeks-plus-days must be converted consistently.
1 day
63 versus 64 days. With reliable LMP and no other concern, this does not support redating.
>7 days
Exactly seven days does not exceed the usual threshold. Eight days does.
The earliest window differs: before 9w0d, a discrepancy of more than five days supports redating. Clinical assessment of LMP reliability still matters.
Case 4 · Size versus age
Hypothetical case: 32w0d by reliable early dating; current biometry corresponds to about 29 weeks.
Retain the established EDD. Confirm technique and assess estimated fetal weight (EFW), abdominal circumference (AC), interval growth and the clinical picture.
SMFM defines FGR as EFW or AC below the 10th percentile for gestational age. “Measuring three weeks behind” alone does not establish that diagnosis.
Escalate suspected growth restriction for provider-directed assessment, including umbilical artery Doppler and surveillance when indicated. Redating can hide the abnormality.
Special situation · ART
Use embryo age and transfer date to assign the EDD.
+261 days
Transfer date to estimated delivery.
+263 days
Transfer date to estimated delivery.
Verify the actual ART record. Review a CRL discrepancy with the provider; do not reflexively replace ART dating or diagnose loss or aneuploidy from size discordance alone.
Special situation · Twins
Measure each fetus and preserve the evidence for the pregnancy-level EDD.
ISUOG recommends the larger CRL for dating in the first trimester. Record both measurements and establish chorionicity and amnionicity.
Use the ART-derived date. Do not create separate EDDs for each twin from their individual sizes.
Keep fetal labels consistent. Size discordance deserves assessment; assigning a younger date to the smaller twin can obscure the difference.
Special situation · Suboptimal dating
Suboptimally dated: no ultrasound confirmed or revised the EDD before 22w0d.
No elective delivery based on uncertain dating. Medically indicated care still follows the best clinical estimate and the clinical indication.
Consider interval ultrasound in 3–4 weeks. Consider surveillance at 39–40 weeks; delivery at 41 weeks is indicated using the best estimate.
Document uncertainty and individualize the plan. A repeat scan evaluates interval growth; it does not recover first-trimester dating precision. Routine amniocentesis for lung maturity is not recommended.
Document · Proposed template
Suggested documentation template for a new-visit dating review.
“EDD: [date].”
This does not show whether menstrual history, outside studies or conflicting dates were reviewed.
LMP: [date / unknown]; certainty: [ ]. Cycles: [usual length, range, regularity]. Relevant history / ART: [ ]. Earliest report: [source, scan date]. US: [CRL/biometry, age, adequacy]. Comparison: [LMP/assigned age; Δ days]. Final EDD: [ ]; basis / rationale: [ ]. Provider review / communication: [ ]. Missing evidence and follow-up: [ ].
Document why the date was retained as well as why it was changed. Keep the ultrasound report and clinical chart aligned.
Communicate · Proposed handoff
Suggested division of work; follow local scope and reporting policy.
Collect history, document measurements and sources, flag missing or conflicting evidence.
Review the discrepancy, assign or confirm the EDD, record the clinical rationale and plan.
Update the shared record, inform the referring team and explain the basis of the final date.
Escalate poor image quality, unresolved source conflicts, ART discrepancies and concerning size findings. Avoid reassuring that “everything is normal” on dating information alone.
Quality improvement · Local proposal
Proposed local quality-improvement measures, not national performance mandates.
Among new visits, how many document LMP certainty and cycle pattern, or why the history is unavailable?
How often is the earliest report identified? Track unresolved requests and their follow-up.
Can a reviewer reconstruct why the EDD was retained or changed?
Do the ultrasound report, clinical chart and referring team use the same reconciled EDD?
Define a sample and denominator, measure a baseline, then choose local improvement targets. Audit late changes for rationale rather than assuming every change is wrong.
Clinical pearls
Six habits worth carrying into the next new visit.
Evidence & controversies
Guidelines support consistency; they do not remove every source of uncertainty.
Early ultrasound is more precise than late dating; reliable early dates should anchor later growth assessment.
Cutoffs combine evidence and expert judgment. Uncertain LMP, late first scans and inadequate records require individualized interpretation.
Study whether structured histories and report retrieval improve documentation and reduce inappropriate EDD changes.
The five-step workflow and audit measures are educational proposals. Their effect on patient outcomes has not been established by this presentation. Twin dating recommendations also include lower-certainty evidence (ISUOG grade C for larger CRL).
Evidence library
Primary guidance supporting the dating and imaging decisions in this module.
Sources checked October 21, 2026. Educational material supports clinical judgment and local policy. Cases are hypothetical; no patient information is included.