The Biparietal Diameter (BPD) is a standard metric used in fetal biometry, the practice of measuring the fetus during a prenatal ultrasound. This measurement is a routine component of prenatal care and helps doctors assess the baby’s growth and development throughout pregnancy. BPD simply represents the width of the baby’s head.
Defining Biparietal Diameter and Measurement Technique
The Biparietal Diameter is the linear distance measured across the widest part of the baby’s skull, specifically between the two parietal bones on either side of the head. This measurement is taken during a standard transabdominal ultrasound scan, which provides a cross-sectional view of the fetal head. The sonographer carefully positions the probe to capture a specific anatomical plane that includes landmarks like the thalami and the cavum septum pellucidum, ensuring a symmetrical image.
To obtain the measurement, electronic calipers are placed on the ultrasound image, typically from the outer edge of the skull closest to the probe to the inner edge of the skull farthest from the probe. This technique, known as the “leading edge” method, is standardized to ensure consistency with existing growth charts. The BPD is most frequently and reliably measured during the second trimester of pregnancy, generally between 14 and 20 weeks.
The Primary Purpose of BPD Assessment
The primary purpose of measuring BPD is to determine or confirm the estimated gestational age of the fetus, which is especially important if the date of conception is uncertain. Since the fetal head grows along a consistent curve, the BPD measurement can be correlated with established nomograms to estimate how far along the pregnancy is. This dating is most accurate when performed in the second trimester, offering a reliable prediction for the due date.
BPD is also a fundamental tool for monitoring fetal growth over time, particularly when comparing multiple scans performed weeks apart. Tracking the progression of the BPD helps ensure that the baby’s head is developing at an expected rate. This head measurement is often combined with other biometric data, such as the Head Circumference (HC), Abdominal Circumference (AC), and Femur Length (FL), to calculate an overall estimated fetal weight.
Interpreting BPD Results and Gestational Age
Once measured in millimeters, the BPD value is plotted onto a specialized growth chart or nomogram corresponding to the stage of pregnancy. This comparison allows the healthcare provider to assign a percentile rank to the measurement, indicating how the baby’s head size compares to others at the same gestational age. A measurement that falls between the 10th and 90th percentiles is typically considered within the expected range for normal growth.
A BPD measurement that is significantly smaller than the expected range may signal a potential issue with fetal growth or development. For example, a disproportionately small BPD could be an indicator of microcephaly. Conversely, a BPD that is significantly larger may suggest a larger-than-average head size, sometimes associated with conditions like hydrocephalus.
The BPD is only one data point, and a single measurement outside the average range does not automatically mean a problem exists. The clinical significance of any deviation is determined by looking at the entire set of biometric measurements and monitoring the growth trend over subsequent appointments.
Factors Influencing BPD Accuracy
The BPD measurement is not equally reliable throughout the entire pregnancy. Its accuracy in estimating gestational age decreases considerably in the third trimester because the rate of head growth begins to vary more significantly between individual fetuses. After about 20 weeks, other measurements, like Femur Length, may offer a more consistent estimate of age.
The baby’s position during the ultrasound can also affect the precision of the measurement. If the head is positioned awkwardly or too low in the pelvis, obtaining the precise cross-sectional view required for an accurate BPD can be challenging.
Furthermore, the natural shape of the baby’s head can influence the BPD reading. For example, a condition called dolichocephaly, where the head is naturally long and narrow, can cause the BPD to be artificially small. Conversely, a head that is naturally wide and short, known as brachycephaly, can result in an artificially large BPD reading. In these instances of variant head shapes, the Head Circumference (HC) measurement is often considered a more reliable indicator of head size and brain development.