Obstetrical Sonography: Fetal Head and Face

Prosencephalon

The forebrain segment of the primitive brain that develops from the rostral end of the neural tube.

Mesencephalon

The midbrain segment of the primitive brain that differentiates from the neural tube around six menstrual weeks.

Rhombencephalon

The hindbrain segment seen on ultrasound as an anechoic structure in the fetal head, which gives rise to the metencephalon and myelencephalon.

Metencephalon

A brain division originating from the rhombencephalon that gives rise to the upper portion of the fourth ventricle, the pons, and the cerebellum.

Myelencephalon

A brain division originating from the rhombencephalon that gives rise to the medulla and the lower portion of the fourth ventricle.

Corpus Callosum

A thick bundle of nerve fibers representing the largest collection of white matter tissue, which connects the left and right cerebral hemispheres.

Cerebrospinal Fluid (CSF)

A clear, colorless fluid produced in the choroid plexus that cushions the cortex, provides mechanical and immunological protection, and plays a vital role in cerebral blood flow autoregulation.

Cephalic Index (CI)

A calculated ratio defined as $\text{CI} = \frac{\text{BPD}}{\text{OFD}} \times 100$ used to evaluate fetal head shape.

Dolichocephalic

A fetal head shape classification corresponding to a cephalic index range of $< 74.9$.

Mesocephalic

A normal fetal head shape classification corresponding to a cephalic index range of $75 - 79.9$.

Brachycephalic

A fetal head shape classification corresponding to a cephalic index range of $80 - 84.9$.

Hyperbrachycephalic

A fetal head shape classification corresponding to a cephalic index range of $> 85$.

Microcephaly

A neurological condition diagnosed when the fetal head perimeter measures two to three standard deviations or more below the mean for gestational age.

Macrocephaly

A condition defined by a fetal head circumference measuring two to three standard deviations above the mean for gestational age and sex, often caused by intracranial anomalies such as hydrocephalus.

Cisterna Magna

A posterior fossa fluid space measured anteroposteriorly in the midline between the vermis and the inner table of the occipital bone, considered normal when measuring $< 10\,mm$.

Anophthalmia

An orbital anomaly characterized by the complete absence of the orbit, optic nerve, chiasma, and optic tracts.

Microphthalmia

An anomaly where the fetal orbital diameter measures smaller than the 5th percentile.

Alobar Holoprosencephaly

The most severe form of holoprosencephaly featuring a single common ventricle, prominent fused thalami, a crescent-shaped frontal cortex, and complete absence of the falx cerebri, corpus callosum, and interhemispheric fissures.

Semilobar Holoprosencephaly