The Biparietal Diameter, or BPD, is a specific measurement routinely obtained during prenatal ultrasound examinations. This value provides a standardized assessment of the developing fetus, helping healthcare providers monitor growth and development throughout the pregnancy. Understanding what constitutes a “normal” BPD measurement requires context about what the measurement represents and how it changes over time.
Defining Biparietal Diameter
The Biparietal Diameter is the measurement of the largest width of the fetal head, specifically the distance between the two parietal bones of the skull. A sonographer obtains this measurement by placing electronic calipers on the outer edge of the proximal parietal bone to the inner edge of the distal parietal bone.
This measurement is a component of a larger set of fetal biometry, which includes the Head Circumference (HC), Abdominal Circumference (AC), and Femur Length (FL). BPD is particularly valuable because it reflects the growth of the fetal skull and, by extension, the brain. The primary goal of capturing the BPD is to estimate the fetus’s overall size and to assess the symmetry of its growth relative to the expected gestational age.
Understanding Normal BPD Growth Curves
What is considered a normal BPD measurement is not a single number but a range that changes progressively with each week of gestation. Healthcare professionals compare the measured BPD against standardized growth charts to determine if the fetus is growing appropriately.
The concept of a normal range is defined statistically, typically spanning from the 5th to the 95th percentile for a given gestational week. For instance, at approximately 20 weeks of gestation, the BPD typically measures between 4.4 and 4.7 centimeters.
The rate of growth is rapid during the second trimester, with the BPD increasing consistently week over week. By around 28 weeks, the BPD will typically have increased to a range of 7.2 to 7.6 centimeters. This steady, predictable increase over time is the most important indicator of healthy neurological development.
Growth curves show that the rate of BPD increase slows down as the pregnancy approaches term, reaching approximately 9.5 centimeters around 40 weeks. The BPD is also subject to slight biological variations, such as fetal sex, with male fetuses often having marginally larger BPD measurements than female fetuses throughout gestation. Evaluating the BPD as part of a trend, rather than as an isolated reading, allows for a more accurate assessment of the fetus’s growth trajectory.
BPD’s Role in Estimating Gestational Age
One of the most frequent applications of the BPD measurement is to estimate or confirm the Estimated Date of Delivery (EDD). Since the growth of the fetal head is highly predictable in early pregnancy, BPD correlates strongly with gestational age during the second trimester. This measurement is most accurate for dating purposes when taken between approximately 14 and 22 weeks of gestation.
In this early window, the variability in fetal head size is relatively minimal, allowing BPD to estimate the gestational age within a narrow margin of error. If a pregnancy has not been dated with an earlier first-trimester ultrasound, the BPD, along with other biometry, serves as the most reliable tool for establishing a due date. This chronological assessment is foundational for managing the rest of the pregnancy.
The reliability of BPD for dating significantly decreases in the third trimester, typically after 26 to 30 weeks. As fetuses grow, natural biological variations in size become more pronounced, meaning two fetuses of the exact same gestational age can have different BPD measurements. Furthermore, the fetal head can undergo a temporary change in shape, known as molding, late in pregnancy, which can artificially alter the BPD measurement.
After 30 weeks, the BPD is used primarily to monitor growth and size rather than to refine the estimated due date.
What Deviations in BPD Measurement May Indicate
When the BPD measurement falls significantly outside the normal range—either below the 5th percentile or above the 95th percentile—it may warrant further investigation. A BPD measurement that is smaller than expected could be a sign of Fetal Growth Restriction (FGR). Certain conditions, such as microcephaly, which involves an abnormally small head, may also be suggested by a significantly low BPD.
Conversely, a BPD that measures larger than the 95th percentile could indicate a larger-than-average head size, which may be a normal familial trait or a sign of conditions like macrosomia. Rarely, an abnormally large BPD may be associated with an excess accumulation of cerebrospinal fluid, known as hydrocephalus. It is important to note that a single outlying measurement does not automatically mean there is a problem.
The BPD is always interpreted in conjunction with the other biometric measurements, particularly the Head Circumference (HC) and Abdominal Circumference (AC). For example, if the BPD and HC are small, but the AC is normal, it might suggest a specific pattern of growth asymmetry. Ultimately, any measurement falling outside the normal range necessitates a thorough clinical correlation and additional monitoring by a healthcare provider.