The Occipitofrontal Diameter (OFD) is a fundamental measurement taken during a prenatal ultrasound to assess the development and size of a fetus. It is a key component of fetal biometry, providing information about the length of the fetal head. Healthcare professionals use this standard metric to monitor the baby’s growth trajectory throughout the pregnancy.
Defining the Occipitofrontal Diameter (OFD)
The Occipitofrontal Diameter is defined as the longest linear distance across the fetal head. It is measured from the outer edge of the frontal bone (forehead) to the outer edge of the occipital bone (back of the skull), representing the front-to-back length of the fetal cranium.
To obtain an accurate OFD measurement, the sonographer must locate the correct anatomical plane, typically the trans-thalamic view. This axial cross-section shows midline structures like the falx cerebri, the cavum septum pellucidum, and the paired thalami. The ultrasound beam should be positioned perpendicular to the central axis of the head. Electronic calipers are then placed on the outer borders of the skull at its longest axis to capture the OFD.
The Role of OFD in Fetal Assessment
The clinical utility of the Occipitofrontal Diameter lies in estimating Gestational Age (GA) and calculating the Estimated Fetal Weight (EFW). OFD is routinely performed in the second and third trimesters to track the baby’s size and growth. By plotting the measurement on standardized growth charts, clinicians determine if the head size is appropriate for the stage of pregnancy.
OFD helps monitor the fetal growth trajectory, allowing for the identification of potential growth issues. Measurements consistently below the expected range might raise concern for Fetal Growth Restriction (FGR), while excessively large measurements could suggest macrosomia. OFD is strongly correlated with gestational age, making it a reliable parameter for dating the pregnancy when used with other biometrics.
How OFD Relates to Other Fetal Head Measurements
OFD is rarely used in isolation; it works in conjunction with the Biparietal Diameter (BPD), the side-to-side measurement across the widest part of the head. BPD is measured perpendicular to the OFD in the same axial plane. The relationship between these two diameters is used to derive the Head Circumference (HC), a more comprehensive measure of head size.
HC can be calculated by tracing an ellipse around the outer border of the skull or estimated using a formula based on the two diameters. A common method calculates HC approximately as \(1.62 \times (\text{BPD} + \text{OFD})\). Combining these two measurements is more robust than using BPD alone for estimating gestational age or fetal size, particularly when the head shape is not spherical.
The Cephalic Index (CI) is another ratio derived from these measurements, calculated as \((\text{BPD} / \text{OFD}) \times 100\). The CI standardizes the head size relative to its shape, with a mean value around 78%. This index identifies whether the head is relatively long or wide compared to the average fetal head shape.
Interpreting OFD Variations and Head Shape
The Cephalic Index is important when the fetal head deviates from the typical oval shape. Since ultrasound formulas rely on the assumption of a standard head shape, a BPD-only measurement can be inaccurate when the shape is distorted. Variations in head shape are classified into two main types that alter the relationship between BPD and OFD.
Dolichocephaly
Dolichocephaly describes a long and narrow head. This shape is characterized by a low BPD and a normal or slightly increased OFD, resulting in a low Cephalic Index, typically below 74%. It is often caused by external factors, such as breech position or lack of amniotic fluid. In these cases, using BPD alone would falsely underestimate gestational age or fetal size, making OFD essential for calculating an accurate Head Circumference.
Brachycephaly
Brachycephaly occurs when the head is short and wide. This shape is identified by an increased BPD and a reduced OFD, leading to a high Cephalic Index, usually above 83%. Brachycephaly can be associated with certain chromosomal abnormalities, but it is also a normal variation in some populations. In both dolichocephaly and brachycephaly, OFD is crucial because the calculated Head Circumference, which integrates both length and width, provides a more reliable assessment of fetal size and growth than the Biparietal Diameter alone.