Fetal biometry is a standard practice in prenatal care, using ultrasound to measure a developing baby’s size and growth. The Bi-Parietal Diameter (BPD) is an important metric, providing a numerical value for the width of the fetal head. This data helps medical professionals monitor the progression of the pregnancy.
Defining the Bi-Parietal Diameter (BPD)
The Bi-Parietal Diameter measures the widest transverse diameter of the fetal skull, specifically the distance between the two parietal bones. It represents the width of the head and is a fundamental component of fetal biometric assessment. This measurement is distinct from the head circumference, which measures the perimeter around the skull.
The BPD is a reliable indicator of fetal head size and the progression of growth. Because the head grows at a predictable rate, tracking this measurement over time helps confirm that development is proceeding as expected. Along with other measurements, the BPD contributes to a comprehensive picture of the baby’s overall physical development.
Ultrasound Technique for Accurate Measurement
The BPD is measured exclusively using prenatal ultrasound, requiring a precise technique for accurate and reproducible readings. The sonographer must first obtain a symmetrical axial plane of the fetal head, which is a cross-sectional view. This specific plane, often called the transthalamic plane, is identified by visualizing specific internal brain structures.
The correct image must symmetrically display the two hemispheres, the midline falx cerebri, and the thalamus on either side of the midline. A small, box-like structure called the cavum septi pellucidi must also be visible in the anterior portion of the brain to confirm the correct plane has been achieved. The ultrasound beam should be held perpendicular to the midline echo, ensuring the measurement is taken across the true width of the head.
Once the optimal image is captured, electronic calipers are placed on the outer edge of the near parietal bone (the one closest to the transducer). The second caliper is then placed on the inner edge of the far parietal bone (the one furthest from the transducer), a technique referred to as the outer-to-inner method. Some international standards now prefer the outer-to-outer method, but the essential goal is to measure the widest perpendicular distance across the skull. Correct placement is essential because an error in the measurement plane can lead to a significant discrepancy in the final value.
Using BPD to Determine Fetal Health and Age
The measured BPD value is a key input for estimating gestational age and assessing fetal well-being, especially in the second trimester. Since the fetal head grows linearly and predictably between 13 and 26 weeks, the BPD value can be plotted onto standardized growth charts to estimate gestational age. During this window, accuracy is typically within 10 to 11 days. BPD is one of the standard biometric parameters, along with head circumference (HC), abdominal circumference (AC), and femur length (FL), used in calculation formulas to estimate fetal weight (EFW).
Tracking the BPD over serial scans allows medical professionals to monitor the growth velocity of the fetal head. A BPD that falls significantly outside the expected range on a growth chart prompts further investigation, as it may signal potential issues with head development. For example, a measurement that is substantially smaller than expected could be an indicator of growth restriction or a condition like microcephaly. Conversely, a significantly larger BPD may indicate macrocephaly or an issue such as hydrocephalus, which might influence delivery planning.
Variables Affecting Measurement Accuracy
While BPD is a valuable tool, its accuracy can be compromised by several biological and technical factors. The reliability of BPD for estimating gestational age decreases considerably after the second trimester. Potential errors can increase to as much as three weeks late in the pregnancy, partly due to the natural variation in fetal head shapes that occurs later in development.
Fetal positioning is a common source of variability, as a compressed or obliquely angled head can distort the measurement. For instance, a fetus in a breech position can sometimes have a flatter head shape, known as dolichocephaly, resulting in a smaller BPD than expected for its age. In such cases, the head circumference is considered a more reliable assessment of cranial growth. Other factors affecting accuracy include the quality of the ultrasound equipment and the skill level of the operator performing the scan.