Normal Fetal Biparietal Diameter
The normal fetal biparietal diameter (BPD) varies with gestational age, ranging from approximately 8 mm at 9 weeks to 95 mm at term, with the measurement most accurate for dating purposes between 11-13 weeks when it can estimate gestational age within ±5-7 days. 1, 2
Gestational Age-Specific Normal Values
First Trimester (9-13 weeks)
- BPD increases from approximately 11 mm at 9 postconceptional weeks to 35 mm by 13 weeks 3
- At 11-13 weeks, BPD measurement provides dating accuracy within ±5-7 days, comparable to crown-rump length (CRL) 4
- The precision of BPD-based dating is optimal at 9-10 completed postconceptional weeks (±4.4 days at 2 standard deviations) 3
Second Trimester (14-27 weeks)
- At 14 completed weeks, BPD measurement accuracy decreases to ±11 days 3
- By 22 postconceptional weeks (approximately 24 weeks gestational age), precision further decreases to ±15 days 3
- BPD measurement at ≥14 weeks is specifically recommended when last menstrual period is uncertain, as it simultaneously rules out anencephaly and improves detection of open spina bifida 2, 5
Third Trimester (28+ weeks)
- A BPD of 8.5 cm corresponds to approximately 32-33 weeks of gestational age in a normally developing fetus 1
- Measurement precision decreases significantly in the third trimester, making BPD less reliable for dating purposes at this stage 1
- BPD continues to increase until term, reaching approximately 95 mm 6
Clinical Applications and Interpretation
Dating Accuracy Hierarchy
- First-trimester CRL remains the gold standard for pregnancy dating (±5-7 days), but BPD at 11-13 weeks provides comparable accuracy 4, 5
- When first-trimester dating is missed, composite ultrasound dating at 18-22 weeks using multiple measurements including BPD provides accuracy within ±10 days 2, 5
- BPD alone should not be used for initial dating in the first trimester when CRL is available, as CRL remains superior 5
Pathologic Considerations
- A discordant BPD measurement compared to expected gestational age should prompt evaluation for skeletal dysplasia, fetal growth restriction, or macrosomia 1
- The femur length to BPD ratio serves as a key predictor of these conditions, particularly at 18-20 weeks gestation 1
- When using BPD to assess for hypoplastic nasal bone (a soft marker for aneuploidy), the ratio is defined as BPD-to-nasal bone of 10 or 11 7
Important Clinical Caveats
Measurement Technique
- BPD must be measured perpendicular to the longitudinal axis in the appropriate plane to ensure accuracy 7
- Proper technique is crucial as measurement errors can significantly affect dating accuracy and subsequent clinical decisions 5
Timing Considerations
- Dating accuracy progressively decreases as pregnancy advances beyond 13 weeks 2, 3
- The imprecision associated with BPD measurements greater than 35 mm (beyond approximately 13 weeks) necessitates earlier ultrasound dating when possible 3
- A recent multicenter study demonstrated that BPD-based dating at 11-13 weeks resulted in 1.3% fewer inductions for post-term pregnancy compared to CRL-based dating, suggesting potential clinical implications of the dating method chosen 4
Growth Monitoring
- Longitudinal BPD measurements are more valuable than single measurements, as individual growth curves reduce variance by 54% compared to population-based curves 6
- Standard fetal biometry assessment should include BPD/head circumference along with abdominal diameter/circumference and femur length for comprehensive growth evaluation 1