Ultrasound scans are a standard part of prenatal care, providing detailed images of a developing baby. These scans involve taking several measurements, which are then used to monitor growth and development throughout the pregnancy. One of the first and most frequently listed measurements on an ultrasound report is BPD, which stands for Biparietal Diameter. BPD provides a specific metric about the size of the baby’s head, helping healthcare providers assess the baby’s well-being.
Defining Biparietal Diameter
The Biparietal Diameter (BPD) is a measurement of the fetal skull, representing the widest transverse diameter of the head. It is the distance measured between the two parietal bones, which form the sides and roof of the skull. BPD is a linear measurement, distinct from the Head Circumference (HC), which measures the perimeter around the skull.
To obtain an accurate BPD, the sonographer locates a specific axial cross-section of the baby’s head. This view must include two anatomical landmarks: the thalami and the cavum septi pellucidi. The resulting image should show an oval-shaped head, ensuring the measurement is taken at the appropriate level.
The standard methodology involves placing electronic calipers on the ultrasound image. The measurement is typically taken from the outer edge of the near skull bone to the inner edge of the skull bone farthest from the probe. This technique accurately reflects the width of the fetal head and is used to assess fetal growth and age.
Clinical Purpose in Prenatal Assessment
BPD serves two primary functions: estimating gestational age and tracking fetal growth. Since the fetal head grows at a predictable rate, BPD provides a reliable metric for estimating how far along the pregnancy is. This is helpful when the date of conception is uncertain or the mother’s menstrual cycle is irregular.
The BPD is most accurate for estimating gestational age during the second trimester, typically between 14 and 26 weeks. After 26 weeks, its reliability for dating decreases significantly as individual fetal growth rates vary more widely. Measurements taken later in pregnancy are primarily used for monitoring growth rather than determining the due date.
BPD is one of four main biometric parameters used in fetal biometry to assess development. It is combined with Head Circumference (HC), Abdominal Circumference (AC), and Femur Length (FL) to estimate the baby’s weight. A consistent increase in BPD over time indicates normal brain and skull development.
Interpreting BPD Measurements
BPD results are compared against established reference charts showing the expected range of measurements for each gestational week. A result falling between the 10th and 90th percentile is considered within the normal range for that stage of pregnancy. Small variations are common and are not typically a cause for concern, as fetal size naturally varies.
High BPD Readings
A BPD measurement significantly higher than expected may suggest the baby is larger than average, a condition sometimes referred to as macrosomia. A high BPD could also be associated with conditions like hydrocephalus or advanced gestational age. Furthermore, a rounder head shape (brachycephaly) may naturally yield a larger BPD measurement without indicating a growth issue.
Low BPD Readings
Conversely, a BPD measurement significantly lower than expected may indicate intrauterine growth restriction or a flatter head shape. A very low BPD may be a sign of microcephaly, which involves a smaller-than-expected head and requires further monitoring. Healthcare providers stress that BPD is only one data point and must be evaluated alongside all other biometric measurements and the overall clinical context. Deviations from the norm usually lead to further investigation, such as repeat ultrasounds.