PREGNANCY DATING SCANS

1. Why is Gestational Dating Essential?
2. How is Gestational Age Estimated? (A Ranking)
3. First Trimester – The Pinnacle of Dating Accuracy
4. Second Trimester – Navigating Challenges
5. Third Trimester – The Unpredictable Phase of Dating
6. Common Mistakes & Prevention Strategies
7. Radiologists' Essential Guidelines for Gestational Dating

Understanding Gestational Dating – The Key to Fetal Evaluation

1. Why is Gestational Dating Essential

Think of gestational age as a navigation tool for the fetus—if the starting point is incorrect, you’ll end up lost.

Having the right gestational age is crucial for scheduling scans, screenings, and delivery accurately.
Getting it wrong leads to misdiagnosing growth issues or large babies, resulting in needless interventions or anxiety.

First-trimester scans are the benchmark (akin to having a reliable GPS—trust it!).

2. How is Gestational Age Estimated? (A Ranking)
Let’s rate the methods from Most Accurate to Least Reliable, similar to evaluating diagnostic tools:

Method	When used	How Accurate	Problems
Crown-Rump Length (CRL)	Timing: 7-10 weeks	Accuracy: ±5-7 days	Issues: Dependent on operator skill, fetal positioning errors
Biparietal Diameter (BPD)	Timing: Second trimester (14-24 weeks)	Accuracy: ±7-10 days	Issues: Variations due to head shape (e.g., breech position)
Head Circumference (HC)	Timing: Second trimester	Accuracy: More reliable than BPD!	Issues: Some growth variation remains
Femur Length (FL)	Timing: Second trimester	Accuracy: ±10-14 days	Issues: Influenced by genetic height differences
Abdominal Circumference (AC)	Timing: Third trimester	Accuracy: Least reliable!	Issues: Affected by fetal weight, growth restriction, maternal diabetes

Key Insight: The first-trimester CRL is the gold standard! Once assigned, don't alter the gestational age.

3. First Trimester – The Pinnacle of Dating Accuracy

Why is First Trimester Dating So Trustworthy?

CRL measures the baby directly (as opposed to the sac, which is like judging someone's size based on their home).
Most precise when assessed between 7-10 weeks.
Once the gestational age is determined, DO NOT MODIFIES it later.

When to Adjust Gestational Age in the First Trimester?
Only adjust if the CRL-based age differs from the Last Menstrual Period (LMP)-based age by:

GESTATIONAL AGE (WEEKS)	REASSIGN IF DIFFERENCE IS MORE THAN…
≤ 9 WEEKS:	>5 DAYS
9 – 13+6 WEEKS:	>7 DAYS

Key Insight: If there's a mismatch between CRL and LMP, trust the CRL measurement.

4. Second Trimester – Navigating Challenges
Why is Dating During the Second Trimester Less Reliable?

Babies begin to grow at varying rates.
Head shape differences can skew BPD readings
Femur length can be influenced by genetics (short parents don’t always mean growth issues!).

Best Biometric Markers in the Second Trimester?
Let’s evaluate them again:

Biometric parameter	How Reliable	Why
Head Circumference (HC)	Most reliable	Why? Less impacted by head molding
Corrected Biparietal Diameter (cBPD)	More dependable than BPD	More dependable than BPD
Femur Length (FL)	Moderately accurate	Why? Genetic height influences results
Abdominal Circumference (AC)	Least accurate	Why? Influenced by fetal weight & maternal diabetes

Key Insight: Use HC and corrected BPD for dating in the second trimester

5. Third Trimester – The Unpredictable Phase of Dating
Why is Third Trimester Dating Often Ineffective? · Babies grow at erratic rates (±21-30 days error margin!).

A larger baby doesn’t necessarily indicate an older age (macrosomia ≠ incorrect dating).

Similarly, a smaller baby doesn’t mean it’s younger (FGR ≠ miscalculated age).
When to Adjust Gestational Age in the Second and Third Trimester?

GESTATIONAL AGE (WEEKS)	REASSIGN IF DIFFERENCE IS MORE THAN…
14 – 16 WEEKS:	>7 DAYS
16 – 22 WEEKS:	>10 DAYS
22 – 28 WEEKS:	>14 DAYS
>28 WEEKS:	>21 DAYS

Key Insight: After 14 weeks, focus on monitoring growth rather than adjusting gestational age!

6. Common Mistakes & Prevention Strategies

Iatrogenic Fetal Growth Restriction (FGR) – When Radiologists Contribute to the Issue

How do we mistakenly diagnose FGR?

By incorrectly dating the pregnancy, making a healthy baby appear smaller than anticipated.
This can trigger unnecessary panic, additional Dopplers, and early deliveries!

Lesson Learned: Review prior dating scans before concluding FGR.

ART (IVF) Pregnancies – Special Considerations

Forget about LMP; use the Embryo Transfer Date (ETD).
For Fresh Embryo Transfers: ETD = DOC (Date of Conception).
For Frozen Embryo Transfers

Day 3 Embryo Transfer: Subtract 3 days from transfer date.

Day 5 Blastocyst Transfer: Subtract 5 days from transfer date
Example:
For a Day 5 blastocyst transferred on December 19, 2023 → DOC = December 14, 2023.

7. Radiologists' Essential Guidelines for Gestational Dating
✔ Use first-trimester CRL for dating & do not alter it afterward.
✔ In the second trimester, monitor growth without changing GA unless there's a major discrepancy.
✔ In the third trimester, avoid redating; concentrate on growth trends instead.

Click here to Join our Advance fetal medicine Course to furhter enhance your knowledge about PREGNANCY DATING

PREGNANCY DATING SCANS

Understanding Gestational Dating – The Key to Fetal Evaluation

Sign Up Complete your profile

Log In

You may also be interested in