The Biparietal Diameter, or BPD, is one of the foundational measurements taken during a routine prenatal ultrasound to assess fetal development. This measurement represents a specific width of the fetal head, providing healthcare providers with quantitative data about the baby’s growth inside the womb. As a standard part of fetal biometry, the BPD is a valuable tool for tracking the progress of the pregnancy.
Defining Biparietal Diameter and Measurement Technique
The Biparietal Diameter is defined as the distance measured between the two sides of the fetal skull, specifically the space spanning the two parietal bones. This measurement is taken across the widest transverse section of the fetal head. Obtaining an accurate BPD measurement requires the sonographer to visualize a precise axial plane of the fetal brain.
The correct imaging plane is identified by several internal anatomical landmarks that must be visible simultaneously. These include the midline falx cerebri, which separates the cerebral hemispheres, and the thalami, which appear as symmetrical, paired structures. The cavum septi pellucidi, a small fluid-filled structure, must also be visible in the anterior brain image to confirm the correct measurement level.
To measure the BPD, electronic calipers are placed on the ultrasound image at the widest part of the fetal head, perpendicular to the midline falx. The standard technique, known as the outer-to-inner technique, involves placing the first caliper on the outer edge of the proximal parietal bone and the second caliper on the inner edge of the distal parietal bone. This ensures consistency by accounting for the thickness of the near skull bone while measuring to the inner surface of the far skull.
The accuracy of this single measurement is highly dependent on achieving the perfect cross-sectional view, as even a slight tilt of the ultrasound probe can significantly distort the reading. An incorrectly angled measurement, or one taken outside the defined transthalamic plane, can lead to an erroneous estimation of the fetal head size. The BPD value is typically recorded in millimeters and then used in calculations that help assess the progression of the pregnancy.
BPD’s Role in Estimating Gestational Age and Fetal Growth
The primary clinical utility of the BPD measurement is its use in estimating the Estimated Gestational Age (EGA) of the fetus. Because the fetal skull grows at a relatively predictable rate during the second trimester, the BPD correlates closely with the age of the pregnancy. This measurement is particularly accurate for dating purposes between 14 and 26 weeks of gestation.
During this window, the BPD is considered one of the most reliable biometric parameters for determining a due date, offering an estimate that is usually accurate within a range of one to two weeks. Once the EGA is established early in the second trimester, it is generally not changed, even if subsequent measurements suggest a different age. The initial dating ensures that all future growth assessments are measured against a fixed timeline.
Beyond dating, the BPD is an important component of assessing fetal growth and size. It is combined mathematically with other standard biometric measurements, including the Head Circumference (HC), Abdominal Circumference (AC), and Femur Length (FL). These four measurements, known collectively as fetal biometry, are used in established formulas to calculate the Estimated Fetal Weight (EFW).
The EFW calculation provides a comprehensive estimate of the baby’s weight, which is then plotted on growth charts to determine if the baby is growing appropriately for their gestational age. While BPD indicates head growth, its reliability decreases in the late second and third trimesters. This occurs because head size variability increases later in pregnancy, and the fetal head often descends into the pelvis, making accurate measurement difficult.
For this reason, the Head Circumference is often favored over BPD for tracking late-stage head growth, as it is less affected by common variations in fetal head shape. However, the BPD remains a standard part of the full biometric profile and is necessary for the most accurate calculation of the Estimated Fetal Weight throughout the entire second and third trimesters.
What Deviations in BPD Measurement May Indicate
When the BPD measurement falls significantly outside the expected range, it may signal a need for further investigation. A substantially smaller BPD can raise concerns about conditions that restrict fetal growth. For example, a reduced BPD may indicate Microcephaly, where the head is significantly smaller than average, or be part of a pattern seen in Intrauterine Growth Restriction (IUGR).
Conversely, a BPD that measures significantly larger than the mean can also indicate underlying issues. An unusually large BPD may be associated with conditions like Hydrocephalus, where excessive cerebrospinal fluid causes the skull to expand. A BPD at the higher end of the normal range, especially when paired with a large abdominal circumference, may contribute to a diagnosis of Macrosomia, referring to a fetus that is much larger than average.
It is important to recognize that BPD can be influenced by simple, normal variations in the shape of the fetal head. If the fetal head is naturally elongated (a shape called dolichocephaly), the BPD will measure smaller than expected, even if the overall head volume is normal. Conversely, a rounder or wider head (brachycephaly) will result in a larger-than-expected BPD measurement.
Because head shape variations can skew the BPD value, clinicians rely heavily on the Head Circumference (HC) and the full set of biometric measurements for diagnosis. The ratio of the BPD to the Occipitofrontal Diameter (OFD), known as the Cephalic Index, helps determine if an abnormal BPD is due to an underlying condition or merely a variant of normal head shape. A diagnosis is never made on the BPD alone but requires a thorough review of the entire fetal biometry profile.