Why This Series?

In day-to-day clinical practice, whether in hospitals, health clinics, or MCHCs, care is delivered within real world constraints. Clinics are busy, manpower is limited, and decisions must often be made efficiently.

Many gaps in patient care do not necessarily arise from lack of knowledge, but also from small variations in routine practice. These are rarely dramatic errors. They are subtle, cumulative, and often system driven.

This series focuses on refining everyday clinical decision making in obstetrics, particularly when and how to apply established standards in common scenarios.

We start off this series revisiting one foundational step: first trimester dating by ultrasound.

First Trimester Dating by Ultrasound: Getting the Basics Right

Scope of This Discussion

This article focuses specifically on when and how to date a pregnancy in the first trimester using ultrasound, once an adequate image has been obtained.
It does not cover:

• Second trimester dating principles
• Assessment of extrauterine pregnancy & miscarriage
• Evaluation of abnormal intrauterine pregnancy
• Technical performance of ultrasound scanning

Hands-on scanning skills require structured training and supervised practice. The focus here is clinical decision making, not probe technique.

1. Timing: When Should Dating Be Done?

Crown rump length (CRL) measurement is the most accurate method for pregnancy dating in the first trimester.

Optimal window:
8+0 to 13+6 weeks
(Most reliable between 9–12 weeks)

Before 8 weeks:

• Visualization may be limited
• Small measurement differences create larger gestational age discrepancies

After 14 weeks:

• CRL is no longer appropriate
• Second trimester biometry is less precise for dating

Key Point:
Early, properly timed dating reduces later uncertainty.

2. Use the Correct Parameter

In the first trimester, CRL alone should be used for dating.
A common pitfall is measuring multiple parameters (BPD, HC, AC, FL) and allowing the machine to average them to generate an EDD.
In early pregnancy:

• Head shape affects BPD reliability
• Limb measurements are not validated for dating
• Automated averaging may introduce avoidable error

Technology should support clinical judgement — not override it.

3. When Should the Pregnancy Be Re-Dated?

For a reliable first trimester scan (≤13+6 weeks):
If ultrasound differs from LMP by:

• >5 days before 9 weeks
• >7 days between 9–13+6 weeks

Adjust the EDD to the ultrasound date.
After 14 weeks:

• Dating precision decreases
• Routine re-dating should be avoided if a reliable early CRL already established the EDD

Key Principle:
Once properly established, the EDD should remain consistent.

4. When the View Is Suboptimal

If a true midsagittal CRL view cannot be confidently obtained:

• Do not accept an off-axis measurement
• Optimise maternal position if appropriate
• Reassess technical factors

If still inadequate, repeat the scan after 5–7 days, allowing growth to improve visualization.

Practical Tip:
A short planned re-scan is preferable to committing to an uncertain measurement.

Avoid repeated postponement beyond 13+6 weeks.

5. Documentation and Quality Control

Good practice extends beyond measurement.
Retaining or printing the CRL image with calipers visible provides:

• Confirmation of correct plane
• Opportunity for peer review
• Protection if discrepancies arise later
• A simple form of internal quality control

Standardisation begins with documentation

6. Common Pitfalls

These may appear minor at the time, but can complicate later management.

7. Closing Reflection

First trimester dating sets the timeline for the entire pregnancy.
Small refinements in timing, parameter selection, re-dating decisions, and documentation reduce avoidable uncertainty later.

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This series is intended to support reflective, practical improvement in everyday obstetric & gynaecology care.
Future topic suggestions, feedback, and clinical questions are welcome.
Shared experiences from hospitals, health clinics, and MCHCs help identify areas where further clarification or discussion may be beneficial.

References

1. National Institute for Health and Care Excellence (NICE).
   Antenatal care (NG201). 2021. Updated guidance on pregnancy dating and ultrasound standards.
2. American College of Obstetricians and Gynecologists (ACOG).
   Committee Opinion No. 700: Methods for Estimating the Due Date. 2017 (Reaffirmed 2021).
3. International Society of Ultrasound in Obstetrics and Gynecology (ISUOG).
   Practice Guidelines: Performance of First-Trimester Fetal Ultrasound Scan. Updated recommendations.

Written by:
Dr Nurulhuda bt Samsudin
Obstetrician & Gynaecologist
Sarawak General Hospital