Podcast Summary: Dr. Chapa’s OBGYN Clinical Pearls
Episode Title: Fetal AC > 90%: Diagnosis?
Date: April 11, 2026
Host: Dr. Chapa
Episode Overview
This episode of Dr. Chapa’s OBGYN Clinical Pearls dives into a common clinical question: What does it mean when a fetus has an isolated abdominal circumference (AC) greater than the 90th percentile but otherwise normal anatomy? Dr. Chapa unpacks the latest evidence and guidelines to clarify diagnosis, management implications, and clinical significance, all in the podcast’s signature conversational and educational style.
Key Discussion Points & Insights
1. Setting the Stage: The Diagnostic Dilemma
- Diagnostic Jeopardy: Dr. Chapa introduces the episode with a playful nod to “Diagnosis Jeopardy”—highlighting that revisiting fundamental diagnostic questions is crucial for learning.
- “Sometimes it's good to just rehash some diagnostic issue or dilemma because it's actually a good learning case.” – Dr. Chapa (01:08)
- A recent resident case prompts the discussion: Isolated fetal AC > 90th percentile, normal anatomy, all other biometric parameters < 90th percentile.
2. What Is the Diagnostic Significance of Isolated Elevated AC?
- Fetal Growth Restriction (FGR):
- Isolated AC < 10th percentile is diagnostic for FGR (per AIUM, ACOG, SMFM, NICE).
- “The abdominal circumference under 10% by itself is diagnostic of FGR.” – Dr. Chapa (02:37)
- Management (e.g., Doppler ultrasound, surveillance) is based on trends in EFW (Estimated Fetal Weight), not just isolated AC.
- Does the opposite hold true?
- The logical (but incorrect) assumption: If a small AC = FGR, does a large AC = LGA (Large-for-Gestational-Age)?
- “So logistically that’s correct. The problem is diagnostically that is not correct.” – Dr. Chapa (05:46)
- Key Point: As of April 2026, isolated AC > 90th percentile alone does NOT diagnose LGA, according to national and international guidelines.
3. Defining LGA and Macrosomia
- Current Standard:
- LGA: EFW > 90th percentile
- Macrosomia: EFW ≥ 4,000 grams (with some policies using ≥ 4,250–4,500 grams, especially with diabetes)
- “Not all LGAs are macrosomic, but all macrosomics are LGA.” – Dr. Chapa (07:37)
- Key Reference:
- “The standard definition of LGA is an estimated fetal weight of greater than the 90th percentile, with Macrosomia reserved for an absolute cutoff of 4,000 grams or more.” – Dr. Chapa (07:36)
4. Clinical Significance of Isolated AC > 90th Percentile
a) Prognostic, Not Diagnostic
- Sensitivity to Future LGA/Macrosomia:
- An isolated AC > 90th percentile (especially at 28–34 weeks) is a strong predictor (high likelihood ratio) for LGA/macrosomia at birth, but not diagnostic.
- Action: Repeat growth ultrasound in ~3 weeks to re-evaluate.
b) Statistics Matter: Not All “Large” Fetuses Become LGA
- Positive Predictive Value (PPV):
- “While AC greater than 90th percentile is not by itself diagnostic of LGA, it could signify greater growth down the line…its positive predictive value in the general population is still relatively poor.” – Dr. Chapa (08:36)
- PPV ranges from 24%–40%.
- Negative Predictive Value (NPV):
- High NPV (~96%); a normal AC makes significant overgrowth unlikely.
- Quote:
- “So much better at ruling it out than it is at ruling it in.” – Dr. Chapa (10:55)
c) Management Implications
- No Immediate Diagnosis or Intervention:
- “It is not a diagnosis for anything. It's just an AC of greater than 90th percent.”
- Flag and Monitor:
- Place a flag in the medical record, inform the patient, and repeat ultrasound to monitor growth trends.
- Prepare for Variables:
- Particularly significant in mothers with hyperglycemia, where risk for true LGA/macrosomia increases.
d) Macrosomia Management Cutoffs (Per National Guidance)
- Without diabetes: Offer C-section at ≥5,000 grams
- With diabetes: Offer C-section at ≥4,500 grams
- Some hospitals may use 4,250 grams (local protocol applies).
Memorable Quotes & Moments
On the Limits of Diagnosis
- “An AC of greater than 90% as a diagnosis is nothing. There is no diagnosis for that. It's just an AC of greater than 90th percent. What is the diagnosis? The diagnosis is it's a big fat abdomen, but there's nothing to do with that.” – Dr. Chapa (12:03)
Take Home Messages (12:00)
- AC > 90th percentile is not diagnostic of anything.
- Don’t ignore it—flag and re-assess with growth scans.
- Reassure the patient—while predictive, it isn’t determinative.
- “Let the patient relax, calm down, it’s all right. Because while the likelihood ratio is there, its positive predictive value when that is the only factor alone over 90th percentile is under 50th percent.” – Dr. Chapa (12:14)
Timestamps for Important Segments
- 01:08 – Setting up the diagnostic scenario (resident question, clinical case)
- 02:37 – Diagnostic criteria for FGR and comparison with LGA
- 05:46 – Why isolated large AC is not diagnostic for LGA
- 07:36 – Definitions: LGA vs Macrosomia
- 08:36 – Clinical management and predictive value of isolated AC > 90th percentile
- 10:55 – The importance of knowing likelihood ratio, PPV, NPV
- 12:00 – Three take-home messages and practical implications
- 13:45 – Management cutoffs for macrosomia / C-section
Conclusion
Summary:
Isolated fetal abdominal circumference > 90th percentile is not in itself diagnostic for LGA or macrosomia but is a strong risk factor for future fetal overgrowth—requiring follow-up but not immediate intervention. Diagnosis of LGA still relies on estimated fetal weight > 90th percentile, and macrosomia at absolute weight cutoffs. Providers should flag and trend the growth, especially noting risks when maternal diabetes is present. The episode underscores the importance of nuanced interpretation of fetal biometry and the need for evidence-based, measured reassurances to patients.
“What is the diagnosis? Nothing. It’s not LGA. It is, however, a flag for impending LGA down the road, but not in and of itself is a diagnostic; the entire EFW has to be greater than the 90th percentile.” – Dr. Chapa (14:28)
End of Summary