Corrected BPD for Gestational Age Calculation
The corrected Biparietal Diameter (BPD) for gestational age calculation involves adjusting the standard BPD measurement to account for variations in fetal head shape, which improves the accuracy of gestational age estimation, particularly for detecting open spina bifida. 1
Understanding BPD Measurement and Correction
Biparietal diameter (BPD) is a key ultrasound measurement used for pregnancy dating, particularly in the first and second trimesters. The standard BPD measurement is taken from the outer edge of the proximal skull to the outer edge of the distal skull (outer-outer measurement).
Why BPD Correction Is Needed:
- In cases of open spina bifida, fetuses typically have BPD measurements that appear equal to a fetus approximately 2 weeks younger 1
- Variations in fetal head shape can affect the accuracy of gestational age estimation
- Maternal and fetal factors can influence BPD measurements by 1-2 days 2
Factors Affecting BPD Accuracy:
- Maternal age and multiparity
- Fetal gender
- Breech position
- Shape of the fetal head 2
How to Calculate Corrected BPD
- Obtain a standard BPD measurement using proper technique (outer-outer measurement)
- Consider the following adjustments:
- If open spina bifida is suspected, recognize that BPD may appear smaller than expected for gestational age (approximately 2 weeks behind) 1
- When using BPD for dating in cases where head shape abnormalities are present, consider using additional measurements
Clinical Application of Corrected BPD
First Trimester Dating:
- BPD is as precise as Crown-Rump Length (CRL) when used for first-trimester dating 3
- BPD has technical advantages as it is less challenging and less time-consuming to measure compared to CRL 3
- BPD can be measured and assessed throughout the entire pregnancy, unlike CRL 3
Second and Third Trimester Dating:
- For improved accuracy, combine BPD with other measurements:
- Head Circumference (HC)
- Femur Length (FL)
- Abdominal Circumference (AC) 4
- Using multiple parameters significantly improves the accuracy of gestational age estimation compared to using BPD alone 5
Special Considerations:
- When dating pregnancies using BPD, ultrasound dating has the effect of "tightening up" the distribution of AFP measurements in both unaffected and affected pregnancies 1
- BPD dating specifically helps rule out anencephaly 1
- For open spina bifida cases, BPD dating improves detection rates at any screening cut-off level due to the characteristic smaller head size 1
Reporting Standards
- Gestational age should be reported as the number of weeks plus the number of days (e.g., 12 weeks 3 days) 4
- For research and standardized reporting, gestational age at birth should be reported as median and interquartile range, or mean and standard deviation 1, 4
Common Pitfalls to Avoid
- Relying solely on BPD for dating after 20 weeks of gestation, when variability in head shape becomes more pronounced 5
- Failing to consider maternal and fetal factors that may affect BPD measurements 2
- Not using multiple parameters (HC, FL, AC) when available, especially after 20 weeks 5
- Overlooking the potential impact of fetal position and head shape on BPD measurements
By understanding and applying corrected BPD measurements, clinicians can improve the accuracy of gestational age estimation, particularly in cases where fetal head shape variations or neural tube defects are present.