Podcast Summary: Understanding Fetal Microcephaly

Podcast: Dr. Chapa’s OBGYN Clinical Pearls
Host: Dr. Chapa
Episode Date: February 5, 2026

Episode Overview

This episode dives into the accurate diagnosis, differential, and management of fetal microcephaly, addressing common misunderstandings and pitfalls for OB/GYN clinicians, residents, and students. Dr. Chapa emphasizes why a nuanced, evidence-based approach is critical, given the potential for life-altering consequences in affected pregnancies. Throughout, the episode retains Dr. Chapa's conversational, engaging tone with memorable clinical pearls and practical insights.

Key Discussion Points & Insights

1. Ultrasound Utility and Diagnostic Pitfalls

[00:35–03:40]

• Ultrasound is the primary tool for fetal assessment in OB/GYN—favored for pelvic and obstetric surveys.
• Occasionally, questionable findings arise, like small head circumference, prompting concern.
• Dr. Chapa notes that percentiles are misapplied—while growth restriction uses <10th percentile, microcephaly is not diagnosed this way.

"Fetal microcephaly is not diagnosed by a percentage, but by a standard deviation from the mean."
— Dr. Chapa [02:50]

• Fetal MRI is highlighted as an adjunct tool, especially when intracranial anomalies are suspected.

2. Defining Fetal Microcephaly: The Standard Deviation Rule

[04:35–10:22]

• Key Definition:
  • Diagnosis hinges on head circumference more than 3 standard deviations (SD) below the mean for gestational age, based on SMFM and major guidelines.
  • Percentiles commonly displayed by ultrasound machines can mislead clinicians.

"Everybody get that? Not percentage. Three standard deviations below the mean."
— Dr. Chapa [07:25]

• Clinical Nuance:
  • <2 SD below mean is notable and warrants follow-up, but does not fulfill diagnostic criteria.
  • At 5 SD below the mean, suspicion for pathological causes heightens; constitutional/familial causes are unlikely.

"If you have one that's five standard deviation below the mean...there's something going on there and that's linked to higher intracranial abnormalities."
— Dr. Chapa [08:55]

3. Accuracy & Limitations of Prenatal Diagnosis

[10:23–13:00]

• Positive Predictive Value: The accuracy of 3 SD cutoff in predicting neonatal microcephaly is about 43–67%, so not all findings translate postnatally.
• Additional Imaging: Abnormal skull shapes/signs (scalloping, sloping, lemon sign) and intracranial abnormalities add predictive value—should be checked in every case.

4. Clinical Pearls and Ratios

[13:01–16:50]

• Gender Differences: Female fetuses typically have slightly smaller head circumferences during fetal surveys, especially between 18–22 weeks.
  • Not a sign of pathology—just a physiological difference.

"Female fetuses generally have smaller head circumferences at a certain gestational age than males."
— Dr. Chapa [14:30]

• Alternative Ratios: Head Circumference:Abdominal Circumference (HC:AC) <3rd percentile can suggest a small head, but often overcalls microcephaly due to low specificity; not recommended for diagnosis.
• Femur length ratios and others are outdated and inaccurate.

5. Etiology Workup: Genetics, Infection, Environment

[16:51–23:30]

• Genetic Causes:

  • Chromosomal (esp. trisomies 13, 18, 21); non-invasive prenatal testing (NIPT) is the first box to check.
  • Single-gene syndromes (e.g., Smith-Lemli-Opitz, Meckel-Gruber) are rare but important.

• Infectious Causes:

  • TORCH infections (Toxoplasmosis, Others, Rubella, CMV, HSV), can cause microcephaly but usually present with additional findings (IC calcifications, eye lesions).
  • Serology interpretation is complex; PCR from amniotic fluid is more definitive. If using antibodies, consider IgM for acute, but pay attention to IgG avidity for infection timing.

"Torch serologies ... are horribly difficult to read ... The best way to figure out if somebody has a TORCH infection ... tap the fluid and send that for PCR."
— Dr. Chapa [19:05]

• Zika: Still present but less prominent today; test in patients with relevant travel history.

• Environmental & Maternal Factors:

  • Alcohol use (fetal alcohol syndrome), severe smoking, and maternal metabolic conditions—notably, poorly controlled phenylketonuria (PKU)—are significant causes.

"PKU needs to be respected in a childbearing woman, because, man, they gotta stick to that."
— Dr. Chapa [22:25]

6. Workup & Next Steps After Suspicion

[23:31–28:30]

• If measurement is 2 SD below mean:
  • Consider genetics consult and trend over time.
• At 3 SD or more:
  • Strongly recommend fetal MRI, especially with no observable gross abnormalities.
  • MRI should be interpreted by a specialist (fetal neuroradiologist if possible).

"MRI detects more things than the ultrasound ... it needs to be somebody at a fetal diagnostic center who knows how to do, ideally, a fetal neuroradiologist."
— Dr. Chapa [25:00]

• Antepartum Surveillance:
  • Reasonable for true microcephaly cases, as CNS abnormalities can affect fetal well-being.
  • Weekly testing (BPP or modified BPP) is suggested; Dopplers may not add significant value.

7. Long-term Management and Prognosis

[28:31–30:15]

• Serial Ultrasound: Track both head circumference and overall growth.
• Continue surveillance even after diagnosis. If a cause hasn't been found, keep searching.
• Prognosis:
  • 3 SD below the mean: Poor outcome risk.
  • 5 SD below the mean: Devastating, strong association with structural abnormalities.

8. Guidelines and Resources

[30:16–31:10]

• Canadian SOGC Committee Opinion No. 380 (2019): Excellent review on investigation and management of prenatal microcephaly.
• Fetal Medicine Foundation: Provides patient-friendly resources for further information.

Notable Quotes & Memorable Moments

• "Fetal microcephaly is not diagnosed by a percentage, but by a standard deviation from the mean." [02:50]
• "At 3 standard deviations below the mean and absolutely devastating at 5 standard deviations below the mean." [28:10]
• "MRI detects more things than the ultrasound ... it needs to be somebody at a fetal diagnostic center who knows how to do, ideally, a fetal neuroradiologist." [25:00]
• "PKU needs to be respected in a childbearing woman, because, man, they gotta stick to that." [22:25]
• "If you find it, keep looking for a cause." [28:40]

Timestamps: Key Segments

• 00:35–03:40: Introduction to ultrasound, pitfalls of head circumference percentiles
• 04:35–10:22: Definition using standard deviation, explanation of diagnostic process
• 13:01–14:30: Gender differences in fetal head circumference
• 16:51–23:30: Genetic, infectious, environmental causes
• 23:31–28:30: Recommended workup, fetal MRI, role of specialists
• 28:31–31:10: Surveillance strategy, guidelines, and take-home points

Conclusion: Dr. Chapa’s Clinical Pearls

• Microcephaly seems rare but requires a precise, multi-disciplinary, and vigilant approach.
• Diagnosis must be by standard deviation, not percentiles—3 SD is the cutoff; 5 SD signals severe pathology.
• Carefully consider etiology, initiate appropriate workup (genetics, infections, maternal exposures), and add MRI as needed.
• Ongoing monitoring is key, as is clear patient counseling and documentation.
• Refer to SOGC Committee Opinion No. 380 and the Fetal Medicine Foundation for further practice guidelines.

End of Summary