For determination of fetal age (Foetal biometry) the first-trimester scan is most dependable when it measured the crown-rump length (CRL) valid up to 12 weeks. At 8 weeks of gestation, the diameter of the yolk sac equals the diameter of the head. Some studies also use the diameter of the gestational sac for pregnancy dating. However, since usually ultrasound is done later than the first trimester, parameters used for foetal age and growth monitoring include:
Fetal History>>>
1. Biparietal diameter (BPD),
2. Femur length (FL),
3. Intracerebellar diameter,
4. Length of other long bones like humorous, radius, tibia,
5. Length of the sole of the foot,
6. Interorbital distance
Growth and weight are usually monitored by:
1. Head circumference (HC)
2. Abdominal circumference (AC)
3. Ratio of HC/AC
4. Femur length
The advent of 3-dimensional ultrasound offers a number of potentially important advantages for biometry. The procedure is more accurate and reliable due to improve the definition of landmarks and the ability to measure true volume.
Normal anatomy of Foetal Biometry
A systematic approach to the sonographic examination of the fetal structures is necessary to reveal the numerous possible abnormalities in this complex system along with growth/age determination.
Foetal head
The Foetal head is most often in an occiput-trans-verse position (i.e. the side of the head lies parallel to the mother's abdominal wall).
The lateral ventricles in Foetal biometry
The lateral ventricles are identified as paired echospared areas within the brain substance. The distal ventricles (i.e the one farthest from the ultrasound transducer) are chosen for study, as reverberation artifacts often obscure the anatomy of the proximal hemisphere. A prominent echogenic area is often seen within the lateral ventricle, which represents the choroid plexus.
The Biparietal diameter (BPD)
Perhaps the most intensely studied transverse section of the fetus is at the level of the biparietal diameter (BPD). Several intracranial landmarks are located at a plane 15° above the canthomeatal line and parallel to the base of the skull💀. The two landmarks most consistently found are the roughly triangular, paired, non-echogenic thalami and two short anterior lines parallel to the midline, known as the cavum septi pellucid. Other structures commonly observed in the same plane and near the midline are from posterior to anterior: the great cerebral vein and its ambient Christian sitting above the cerebellum the midbrain the third ventricle between the thalami, and the frontal horns of the lateral ventricles. Lateral placed, in the same plane, are the spiral hippocampal gyri poatilerioely and the bright paired echoes of the insulae with pulsating middle cerebral arteries, commonly studied for velocimetry since it is comparatively easy to focus.
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Measurement of biparietal diameter (BPD)
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In this plane, the BPF can be reproducibly measured as the distance from the proximal outer table to the distal in a table of the skull. In addition, the head parameter can be determined by direct measurement or calculated by summing the BPD and occipitofrontal diameter (BPD) measured from the midpoint of frontal and occipital echo complex and multiplying by 1.62. The ratio of BPD to OFD defines the cephalic index (normal values 75-78). A high value determines brachycephaly and a low-value dolichocephaly.
The Cerebellum
The Cerebellum may be visualized in a plane parallel to the BPD plane with posterior angulation. Once located, the cerebellar structures are best studied by rotating the Ultrasound transducer 15° father from the Cantho-meatal line. In this plane, the cerebellum (with its brightly echogenic, centrally placed vermis and two relatively non-echogenic hemispheres) may be evaluated and measured.
The Base of the skull
The Base of the skull level may be identified by an echogenic 'X' formed by the lesser wings of the sphenoid bone and the petrous pyramid. These bone ridges mark to anterior, middle, And posterior fossae.
The Face
Unlike the cranium and its contents, which may be studied using an orderly progression of well-defined sonographic planes, characterization of the face requires both ingenuity and good luck on the part of a sonographer. 3-D ultrasound is invaluable to study the face.
The Ear👂
The pinna of the ear and the development of its cartilages have been observed sonographically.
The Lower Face
The nose and the upper lip can be visualized using an oblique transverse scan. This plane may be used in the search for cleft lip and cleft palate.
Foetal weight
Abdominal circumference calculated from two orthogonal abdominal diameters at the level of the intrahepatic portion of umbilical vein and stomach along with BPD computes foetal weight. However, the exact weight is not obtained by any. The accuracy of weight prediction comes with the relative error which is the difference between estimated and actual weights, expressed as a fraction of actual weight.
From a large study of 12,980 obstetric cases, the following values of foetal parameters were calculated in the author's ultrasound laboratory
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