Normal Brain-Stem-to-Occipital-Bone Ratio on Mid-Trimester Fetal Ultrasound
The normal brain-stem-to-occipital-bone (BS/BSOB) ratio on mid-trimester fetal ultrasound is approximately 0.3 or higher, with values below this threshold potentially indicating posterior fossa abnormalities.
Measurement Technique
The BS/BSOB ratio is measured in the mid-sagittal plane of the fetal head, the same view used for nuchal translucency assessment 1. This plane should clearly capture:
- The brain stem (BS)
- The occipital bone
- The fourth ventricle
- The cisterna magna
Calipers are placed to measure the brain stem diameter and the distance from the brain stem to the occipital bone (BSOB), and the ratio is calculated 2.
Normal Reference Values
First Trimester (11-13+6 weeks)
Research demonstrates that the BS/BSOB ratio can be assessed as early as the first trimester:
- Cutoff value: 0.3 - Values at or above this threshold have a 100% negative predictive value for posterior fossa abnormalities 2
- The ratio shows a negative correlation with crown-rump length (CRL), meaning it decreases slightly as gestational age advances: BS/BSOB = 0.747 - 0.021 × CRL (cm) 3
- Normal range demonstrates good reproducibility with intraclass correlation coefficients of 0.939 for intra-operator and 0.772 for inter-operator measurements 3
Clinical Significance
The BS/BSOB ratio is particularly valuable for:
Early detection of aneuploidies - Fetuses with trisomy 18 (44% below 5th percentile), trisomy 13 (50%), and monosomy X (50%) show significantly reduced BS/BSOB ratios, though trisomy 21 typically does not 4
Predicting posterior fossa malformations - Abnormal ratios in the first trimester can predict posterior fossa anomalies that become more apparent in later gestational weeks 2, 5
Screening for open spina bifida - Assessment of posterior fossa anatomy, including the BS/BSOB ratio, enhances early identification of Chiari II malformation associated with spina bifida 6
Important Caveats
While the BS/BSOB ratio is useful for screening, definitive diagnosis of posterior fossa malformations (such as Dandy-Walker malformation) typically requires follow-up imaging in the second trimester when structures are more fully developed 5
The measurement requires adequate visualization in the exact mid-sagittal plane - suboptimal imaging can lead to inaccurate measurements 3
Individual measurements of BS and BSOB also correlate with gestational age, so the ratio provides a gestational age-independent marker that may be more clinically useful than absolute measurements alone 2, 3