Pregnancy ultrasounds provide a comprehensive view of a developing fetus, and the reports often contain a series of measurements used to assess growth and estimate age. One of the most common metrics listed is the Biparietal Diameter (BPD), which is a measurement of the fetal head. The term “BPD Hadlock” refers to this specific head measurement being processed through a standardized set of formulas developed by Dr. Frank Hadlock and his colleagues. These formulas translate the raw ultrasound data into crucial clinical estimates, such as the Estimated Gestational Age (EGA) and the Estimated Fetal Weight (EFW). This integration of BPD with the Hadlock mathematical model allows healthcare providers to monitor the baby’s development against established growth charts.
Understanding Biparietal Diameter (BPD)
Biparietal Diameter (BPD) is the measurement of the widest part of the fetal head, specifically the distance between the two parietal bones of the skull. This measurement is obtained by the sonographer on an axial plane of the fetal head that includes specific anatomical landmarks, such as the thalami and the cavum septum pellucidum. The measurement is taken perpendicular to the midline of the head. Modern guidelines recommend placing the calipers from the outer edge of the near skull wall to the outer edge of the far skull wall to ensure standardization.
BPD is considered a highly reliable metric for estimating gestational age, particularly when measured between 14 and 20 weeks of pregnancy. During this mid-trimester period, the fetal head shape is consistently symmetrical, and its growth rate is highly predictable. The BPD value increases steadily throughout pregnancy, starting around 2.4 centimeters at 13 weeks and reaching approximately 9.5 centimeters at term.
The accuracy of BPD in predicting gestational age starts to decrease after 20 weeks as individual variations in head shape become more pronounced. This measurement is only one component of a full fetal biometry assessment, which also includes Head Circumference (HC), Abdominal Circumference (AC), and Femur Length (FL).
The Purpose and Precision of Hadlock Equations
The Hadlock equations represent a standardized mathematical framework developed to convert raw fetal biometric measurements into clinically useful values. Developed in the 1980s by Dr. Frank Hadlock and his team, these regression-based formulas are considered the gold standard for estimating fetal size and age. The purpose of using this specific set of equations is to ensure consistency in reporting and monitoring fetal growth across different medical centers.
The BPD measurement is incorporated into various Hadlock formulas, often alongside other parameters like Head Circumference and Femur Length, to calculate the Estimated Gestational Age (EGA) and Estimated Fetal Weight (EFW). For example, one variation of the Hadlock formula for EFW uses BPD, Abdominal Circumference, and Femur Length in a complex logarithmic equation to predict the baby’s weight. By combining multiple measurements, the Hadlock method mitigates the error that would result from relying on a single data point, accounting for natural variations in fetal proportions.
These formulas are based on large studies of fetal measurements, providing a statistically sound estimate of fetal size. While the Hadlock method is accurate, it is most precise in the mid-trimester for dating the pregnancy. The predictive accuracy of the Estimated Fetal Weight tends to decrease as the baby grows larger, particularly in the third trimester. This is due to biological variability and the difficulty of accurately measuring the fetus late in pregnancy.
Interpreting BPD Measurements in Context
A raw BPD measurement must be interpreted in the context of the Hadlock-derived growth charts and the fetus’s other measurements. The primary way this is done is by calculating the BPD percentile for a given gestational age. For instance, a BPD in the 50th percentile means that the measurement is larger than 50% of fetuses at the same stage of development and smaller than the other 50%.
The range between the 10th and 90th percentiles is considered the normal range for fetal growth. If the BPD falls significantly below the 10th percentile, it may suggest a smaller-than-average head size, which can be associated with conditions like microcephaly or simply a naturally smaller baby. Conversely, a BPD above the 90th percentile suggests a larger head, which can be seen in cases of fetal macrosomia, possibly linked to maternal gestational diabetes, or hydrocephalus.
The BPD is only one part of the overall assessment. If the BPD is small, but the Head Circumference (HC) is normal, the unusual BPD may simply reflect a non-spherical head shape, known as dolichocephaly, which is a normal variation often seen in breech presentations. The interpretation requires a holistic view, comparing the BPD to the HC, AC, and FL, and tracking these measurements over time to determine the fetal growth trajectory. An isolated measurement outside the normal range is usually a prompt for increased monitoring, not an immediate diagnosis of a problem.