Normal Cephalic Index Size on Obstetric Ultrasound
The cephalic index (CI) is a dimensionless ratio of the fetal biparietal diameter (BPD) to the occipitofrontal diameter (OFD), expressed as a percentage: CI = (BPD / OFD) × 100. It quantifies fetal head shape — distinguishing dolichocephaly from brachycephaly — and is essential for validating the reliability of BPD-based gestational age estimates. Accurate CI assessment helps clinicians decide whether BPD alone or head circumference should be used as the primary biometric parameter.
Normal Reference Values
| Measurement |
|---|
| 75-85 |
Clinical Significance
A normal cephalic index of 75–85 indicates an oval, appropriately shaped fetal head. Values below 75 indicate dolichocephaly (elongated head), while values above 85 indicate brachycephaly (shortened, rounded head). In either case, the BPD may not accurately reflect true gestational age, and head circumference becomes the preferred biometric measurement.
Head shape abnormalities can be positional (e.g., breech presentation causing dolichocephaly) or may signal underlying pathology. Persistent or severe deviation from the normal CI warrants careful evaluation of fetal anatomy and neurodevelopment.
- Dolichocephaly (CI <75): Breech lie, oligohydramnios, premature rupture of membranes
- Brachycephaly (CI >85): Trisomy 21 (Down syndrome), craniosynostosis, skeletal dysplasias
- Strawberry-shaped skull: Associated with Trisomy 18
- Lemon sign: Seen in open neural tube defects (e.g., spina bifida with Arnold-Chiari II malformation)
- Cloverleaf skull: Thanatophoric dysplasia or severe craniosynostosis
Reference: Merz E. Ultrasound in Obstetrics and Gynecology. George Thieme Verlag. (2005).
Imaging Notes
On obstetric ultrasound, the CI is measured in the standard axial BPD plane, which passes through the thalami and cavum septi pellucidi at the level of the widest transverse diameter of the skull. The BPD is measured from the outer edge of the near parietal bone to the inner edge of the far parietal bone (outer-to-inner convention), while the OFD is measured between the outer edges of the frontal and occipital bones at the same level. Ensure the fetal head is not compressed by the transducer or uterine wall, as this artificially alters the CI.
If the CI falls outside the 75–85 range, substitute head circumference (HC) as the primary parameter for gestational age assessment, since HC is shape-independent. Always document the CI when reporting fetal biometry to flag potential BPD unreliability.