Accurate Gestational Dating

When to Adjust the Estimated Due Date

Sonographer Boot Camp

Based on ACOG/SMFM Guidelines

Learning Objectives

  • Apply ACOG Committee Opinion #700 criteria
  • Distinguish when to adjust vs. maintain EDD
  • Master trimester-specific measurement standards
  • Recognize clinical decision thresholds

LMP Dating: Three Assumptions

Regular Cycle

28 days ± 3 days

Ovulation Day

Day 14 of cycle

Accurate Recall

Patient knows first day

Reality: Only 40-45% of patients meet all criteria

Why LMP Fails

Patient Factors

  • Uncertain recall (30-40%)
  • Irregular cycles
  • Recent hormonal contraception
  • Breastfeeding amenorrhea

Physiologic Factors

  • Implantation bleeding mistaken for LMP
  • Early pregnancy loss
  • Ovulation timing variation
  • Long or short follicular phase

Ultrasound Dating Methods

Trimester Parameter Optimal Window Accuracy (±days)
First Crown-Rump Length (CRL) 8-136/7 weeks ±5-7
Second Biparietal Diameter (BPD) 14-276/7 weeks ±7-10
Second Composite biometry 14-216/7 weeks ±10-14
Third Not recommended ≥28 weeks ±21-28

CRL Measurement Standards

Technical Requirements

  • Midsagittal plane of fetal long axis
  • Neutral flexion (neither extended nor flexed)
  • Calipers: crown to rump (exclude limbs, yolk sac)
  • Magnify: fetus fills 75% of image

Gold Standard: CRL 8-136/7 weeks most accurate dating method

First Trimester Adjustment Rules

Gestational Age: Up to 136/7 weeks

Scenario Discrepancy Action
CRL vs. LMP agree ≤5 days Use LMP
CRL vs. LMP agree ≤7 days Use LMP (per ACOG 700)
CRL vs. LMP differ >7 days Change to CRL
Uncertain/unknown LMP Any Use CRL

Second Trimester Adjustment Rules

Gestational Age: 140/7 to 276/7 weeks

Scenario Discrepancy Action
BPD/composite vs. LMP ≤7 days Use LMP
BPD/composite vs. LMP 8-10 days Consider changing to US
BPD/composite vs. LMP >10 days Change to US
Uncertain/unknown LMP Any Use ultrasound

Third Trimester Dating

Critical Rule

Never adjust EDD based on third-trimester ultrasound

Gestational age ≥28 weeks: accuracy ±21-28 days

Exception: No prior dating available at all—use with extreme caution and document rationale

Third Trimester: Biological Variability

Constitutional Factors

  • Maternal height/weight
  • Ethnicity
  • Parity
  • Fetal sex

Pathologic Factors

  • Fetal growth restriction
  • Macrosomia
  • Genetic syndromes
  • Placental insufficiency

Result: Growth assessments ≥28 weeks reflect size, not age

Special Case: Multiple Gestations

Dating Rules

  • Use larger fetus CRL for dating
  • Measure both fetal CRLs separately
  • Document chorionicity and amnionicity
  • Apply same LMP vs. CRL discrepancy rules

Rationale: Larger twin less likely to be growth restricted in first trimester

Assisted Reproductive Technology

IVF/ICSI Dating

Base EDD on:

  • Retrieval date (day 0)
  • Transfer date + embryo age
  • Do NOT use LMP

First Trimester US

Compare CRL to ART date

  • If >5-7 days off: early loss suspect
  • Document discrepancy
  • Consider aneuploidy

Required Documentation

Every Dating Ultrasound Report Must Include

  • LMP date (if known) and certainty
  • Measurement parameters used
  • Gestational age by ultrasound
  • Gestational age by LMP
  • Final assigned gestational age
  • Final EDD
  • Rationale if EDD changed

Common Sonographer Errors

Technical Errors

  • CRL in flexed/extended position
  • Including yolk sac or limbs
  • Off-axis measurement
  • Inadequate magnification

Interpretation Errors

  • Changing EDD in third trimester
  • Using wrong discrepancy thresholds
  • Not documenting LMP certainty
  • Multiple EDD changes

Case Study 1

Patient: Jessica, 28 y/o G1P0

History

Presents at 12 weeks by LMP. Uncertain of exact LMP date.

Ultrasound Findings

CRL measurement: 10 weeks 3 days

Discrepancy: 12 days

Case 1: Management

Correct Action

Adjust EDD to match CRL (10w3d)

Rationale

  • First trimester: discrepancy >7 days → use CRL
  • LMP uncertain increases CRL reliability
  • CRL most accurate dating parameter available

Patient Communication: "The ultrasound is more accurate than your period dates. Your baby is measuring 10 weeks 3 days."

Case Study 2

Patient: Sarah, 32 y/o G2P1

History

Presents at 9 weeks by LMP. Certain LMP. Regular 28-day cycles.

Ultrasound Findings

CRL measurement: 9 weeks 1 day

Discrepancy: 6 days

Case 2: Management

Correct Action

Keep LMP-based EDD (9 weeks)

Rationale

  • First trimester: discrepancy ≤7 days → use LMP
  • LMP certain and reliable
  • 6-day difference within measurement error
  • Ultrasound confirms appropriate growth

Patient Communication: "The ultrasound confirms your dates. Everything is measuring right on track."

Case Study 3

Patient: Emily, 35 y/o G3P2

History

32 weeks by early ultrasound (CRL at 8 weeks: 8w2d). Uncertain LMP.

Current Ultrasound (32 weeks)

Composite biometry: 29 weeks 0 days

Concerning for fetal growth restriction

Case 3: Management

Critical Decision

Do NOT change EDD

Rationale

  • Early ultrasound (8 weeks) established reliable GA
  • Third trimester measurements reflect size, not age
  • 29-week measurement suggests FGR, not dating error
  • Changing EDD would mask pathology

Correct Action

Diagnose FGR. Initiate appropriate surveillance. Maintain 32-week dating.

Communicating Dating Changes

1. Explain

Why ultrasound is more accurate for some patients

2. Reassure

Date changes are common and expected

3. Partner

Accurate dating helps plan optimal care

Key Message: "This is not a problem—it's normal for ultrasound to give us more precise information."

Concerning Scenarios

CRL Smaller Than Expected

  • Consider early pregnancy failure if <2mm growth/week
  • Check fetal heart activity
  • Follow-up scan in 7-10 days

CRL Larger Than Expected

  • Less concerning than smaller
  • Document dating change clearly
  • Consider early ovulation vs. uncertain LMP

Quality Metrics

Metric Target Monitoring Method
Dating US <14 weeks >80% of patients Chart review
Appropriate EDD changes 95% compliance with guidelines Peer review
Third-trimester changes <2% (should be rare) Audit flag
Documentation completeness 100% Template compliance

Decision Algorithm Summary

First Trimester (≤136/7 weeks)

CRL vs. LMP ≤7 days → Use LMP

CRL vs. LMP >7 days → Use CRL

Second Trimester (140/7-276/7 weeks)

US vs. LMP ≤10 days → Use LMP

US vs. LMP >10 days → Use US

Third Trimester (≥28 weeks)

Do NOT adjust EDD

Key Takeaways

  • First trimester CRL is the gold standard
  • Trimester-specific thresholds determine when to adjust
  • Never change third-trimester dates—it masks pathology
  • Document rationale for every EDD assignment
  • Early dating protects patients from misclassification

Why Accurate Dating Matters

Too Early Dating

  • Premature delivery
  • Unnecessary NICU admission
  • Respiratory distress
  • Longer hospital stay

Too Late Dating

  • Delayed induction
  • Stillbirth risk
  • Placental insufficiency
  • Meconium aspiration

Stakes: 1 week error affects timing of delivery, antenatal testing, corticosteroids, growth assessment

References

1. American College of Obstetricians and Gynecologists. (2017). Committee Opinion No. 700: Methods for Estimating the Due Date. Obstetrics & Gynecology, 129(5), e150-e154.

2. Butt, K., & Lim, K. (2014). Determination of gestational age by ultrasound. Journal of Obstetrics and Gynaecology Canada, 36(2), 171-181.

3. Savitz, D. A., et al. (2002). Comparison of pregnancy dating by last menstrual period, ultrasound scanning, and their combination. American Journal of Obstetrics and Gynecology, 187(6), 1660-1666.

4. Society for Maternal-Fetal Medicine. (2020). Electronic address: pubs@smfm.org. Ultrasound in pregnancy.

Questions?

Accurate dating starts with you.

Your measurements guide every clinical decision.

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