Super Subparietal Diameter in Obstetrics
The super subparietal diameter is not a standard obstetric measurement and does not appear in contemporary clinical guidelines or practice—you should focus instead on the established fetal biometric parameters: biparietal diameter (BPD), head circumference, abdominal circumference, and femur length for gestational age assessment and fetal evaluation.
Standard Fetal Head Measurements
The recognized fetal cranial measurements in modern obstetric practice include:
- Biparietal diameter (BPD): The most widely accepted transverse measurement of the fetal head, obtained at a standardized plane through the thalami and cavum septum pellucidum 1
- Occipitofrontal diameter: The anteroposterior measurement of the fetal head 2
- Head circumference: Derived from BPD and occipitofrontal diameter measurements 3
Clinical Application of Fetal Biometry
For comprehensive fetal assessment, measure multiple parameters rather than relying on a single measurement. 4
Gestational Age Estimation
- Prior to 36 weeks: Use BPD, abdominal circumference, and femur length in combination 4
- After 36 weeks: Use head circumference, abdominal circumference, and femur length for optimal accuracy 4
- The 18-20 week ultrasound remains the most reliable time to detect skeletal dysplasias and establish accurate dating 3
Fetal Growth Assessment in Hypertensive Disorders
When preeclampsia is diagnosed, perform comprehensive fetal biometry including 3:
- Biparietal diameter with head circumference
- Abdominal circumference
- Femur length (computed to estimate fetal weight)
- Amniotic fluid volume assessment
- Umbilical artery Doppler waveform analysis
Skeletal Dysplasia Evaluation
Key measurements for assessing suspected skeletal dysplasia include 3:
- Femur length to abdominal circumference ratio <0.16 (strongest single predictor of lethality)
- Femur length <5th percentile warrants further evaluation
- Chest to abdomen ratio <0.6
- Micromelia 3 standard deviations below the mean
Common Pitfalls to Avoid
- Do not rely solely on BPD after 20 weeks gestation due to variability in fetal head shape 4
- Recognize that measurement errors in the same direction across multiple parameters are unlikely, providing built-in quality control when using multiple biometric parameters 4
- Understand that coronal BPD can substitute for transverse BPD when the traditional view cannot be obtained (direct occiput anterior/posterior positions or engaged head), though head circumference cannot be calculated from this view 5
- Be aware that plain radiographs are not helpful for refining prenatal skeletal dysplasia diagnosis—ultrasound and MRI are preferred 3
Measurement Technique Considerations
The standardized plane for BPD measurement should include 1:
- Visualization of the thalami
- Third ventricle
- Cavum septum pellucidum
This standardization increases measurement reliability, though biologic variation in fetal growth rates remains the predominant source of variability rather than technical errors 1.