What is the average fetal age with a biparietal diameter (BPD) of 8.5 cm?

Fetal Age with Biparietal Diameter of 8.5 cm

A biparietal diameter (BPD) of 8.5 cm corresponds to approximately 32-33 weeks of gestational age in a normally developing fetus.

Interpretation of BPD Measurement

• BPD of 8.5 cm (85 mm) indicates third trimester gestation, typically falling within the 32-33 week range based on standard fetal biometry curves used in obstetric ultrasound 1.

• The biparietal diameter is a standard fetal biometry measurement that includes assessment of the fetal head circumference, used routinely in prenatal ultrasound protocols 1.

• BPD measurements are most accurate for gestational age dating when obtained earlier in pregnancy (before 24 weeks), with precision decreasing as pregnancy advances 2, 3.

Important Clinical Considerations

Measurement Accuracy

• The precision of BPD dating decreases significantly in the third trimester, with variability increasing from ±4.4 days at 9-10 weeks to ±15 days at 22 weeks, and even greater variability beyond that point 3.

• At a BPD of 8.5 cm, the measurement reflects late second to early third trimester, where biological variation in fetal growth rates becomes the predominant source of variability rather than technical measurement error 4.

Clinical Context Matters

• BPD should be interpreted alongside other biometric parameters including abdominal circumference and femur length to provide comprehensive assessment of fetal growth and gestational age 1, 2.

• A standardized anatomic plane must be used for BPD measurement to ensure reliability, though this does not eliminate biological growth variation 4.

Potential Pathologic Considerations

• If this BPD measurement seems discordant with expected gestational age based on dating criteria, consider evaluation for:

  • Skeletal dysplasia (though BPD is typically preserved or even relatively large compared to long bones in many skeletal dysplasias) 5, 6
  • Fetal growth restriction or macrosomia
  • Dating discrepancies requiring reassessment

• The femur length to biparietal diameter ratio is used as a key predictor when evaluating for skeletal dysplasia, particularly at 18-20 weeks gestation 5, 6.

Practical Application

• For clinical purposes, a BPD of 8.5 cm should prompt correlation with:

  • Last menstrual period dating
  • First trimester ultrasound measurements (if available)
  • Other current biometric parameters (abdominal circumference, femur length)
  • Clinical examination findings 2

• If this represents an isolated measurement without prior dating ultrasound, the estimated gestational age of 32-33 weeks carries significant uncertainty (potentially ±2-3 weeks) and should be used cautiously for clinical decision-making 3.