Ultrasound in obstetrics III By Dr. Khattab KAEO Assis. Prof. of Obstetrics and Gynaecology Faculty of Medicine, Al-Azhar University, Damietta

First trimester ultrasound

It was primarily implemented for dating to improve interpretation of biochemi- cal tests. Recently, it has been used to screen for fetal anomalies, detect chro- mosomal abnormalities and determine chorionicity in case of multiple pregn.

Nuchal translucency (NT) It is an echo-free area at the back of the neck. Fluid collects behind the neck partly because the fetus tends to lie on its back and partly because of the laxity of the skin of the neck. Originally, 3 mm thickness was chosen as the cut-off level; recently, however, a percentile for gestational age was developed and the 9th percentile for gesta- tional age was used. The ideal time to look for nuchal translucency is between weeks ’ gestation. Why? This is the time when the fetal lymphatic system is developing and the peripheral resistance of the placenta is high. After 14 weeks the lymphatic system is developed enough to drain any accumulating fluid and the placental peripheral resistance is dropping. CVS should be offered.

NT

Diagnostic value There is a 10% risk of major abnormality when the measurement is >3 mm, increasing to 90% at >6 The abnormalities are mainly chromosomal. Abnormal NT of 2.5 mm include 77% of babies with Down syndrome and a similar percent of babies with other chromosomal abnormalities. A normal measurement does not guarantee normality. Trisomy 21: Chromosome 21 carries the gene that is codes for type VI collagen. Trisomy 21  over- expression of one subunit of this collagen  CT that has more elastic composition. Neuromuscular abnormalities  poor breathing and body movements  oedema. Intra- and extra-thoracic compressive syndromes  congestion of the vessels in the fetus ’ s head and neck. Heart failure, due to congenital cardiac abnormal- ity or cardiac dysfunction.

Prerequisites - The measurement is done at w. - Sagittal view of the fetus. - Neutral position between head flexion and extension. - The maximum transluscent distance is measured. - The amnion should be identified separately from the skin.

The detailed anomaly scan This is normally undertaken between 18 and 20 weeks' gestation.

The head: The following structures are to be examined: the skull bones for their integrity, the brain, the orbit and the lips. Facial profile and the ears are examined too.

The brain: Within each cerebral hemisphere is the lateral vent- ricle which contains the choroid plexus (a vascular echogenic structure which produces most of the CSF). The third ventricle is a central structure narrowed by the development of the thalamus on either side. The 4th ventricle lies in the hindbrain which lies anterior to the cerebellum.

Then the transcerebellar plane should be obtained. Originating from the transthalamic plane the transducer is rotated until the cerebellar hemispheres (double head dumb-bell like structures) are viewed.

Abnormal transcerebellar view A cerebellum which is banana shaped is associated with spina bifida. The transcerebellar distance (TCD) is measured from the outer edge of one hemisphere to the outer edge of the other. Hypoplasia (small TCD) may be associated with spina bifida. The cisterna magna (a space posterior to the cerebellum) is usually not greater than 10 mm. It is usually very enlarged with the Dandy-Walker malformation which is associated with cerebellar hypoplasia.

The transventricular plane It is used to assess the lateral vent- ricles to determine if there is ventriculo megally or choroid plexus cysts. Originating from the transthalamic plane the transducer is rotated until the choroid plexus is identified. This arises from the medial wall of the vent- ricle and extends into the poster horn.

Choroid plexus cysts may be uni- or bilateral. Early cases of ventriculomegally may be identified by a >3 mm separation of the choroid plexus from the ventricular wall. In late cases, the choroid plexus will appear like the clapper of a bell (gravity dependent).

Early ventriculomegally

Choroid plexus cyst

Face

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