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    A Role for Ultrasound: Earlier in the first Trimester.

    www.gehealthcare.com
    Ultrasound Post.
    Technical development and medical research - news and facts
    Dr. Lou PISTORIUS, Department of Obstetrics at the University Medical Centre Utrecht in the Netherlands.

    Patient demand usually follows hot on the heels of each technological advance, and prenatal diagnosis is no exception. The last decades have seen screening for chromosomal abnormalities brought forward, from around 16 weeks with the triple test to around 12 weeks with a combination of biochemical and ultrasound screening. Women certainly prefer earlier screening for Down syndrome. However, earlier screening will also detect more abnormalities in fetuses which are destined to abort.

    Where ultrasound screening has been brought forward to the late first/early second trimester (11 -14 weeks), detection rates for structural abnormalities fall to around half those of screening in the second trimester. Exceptions are detection rates for anencephaly, holoprosencephaly, encephalocele, omphalocele and gastroschisis, at over 75% at 11- 14 weeks. When screening even earlier, in the late embryonic period (up to 10 weeks gestation), the a priori risk of abnormalities certainly increases - for example, neural tube defects occur in around 1% of spontaneous miscarriages, about 10 times the risk at term.

    The smaller size of the conceptus during earlier first trimester screening, and its more limited movement compared to 11-14 week or 17-23 week old fetuses, means that only a single 3D volume sweep is required, thereby reducing the embryo's ultrasound exposure. However, in addition to intra-uterine lethality and the greater a priori chance of finding abnormalities, we also need to take into account the rapid changes in appearance of the developing structures.

    The following is a brief review of the anatomical and ultrasound appearance of structures during the late first/early second trimester: (Note: Although the literature on embryogenesis usually refers to postconceptional days, here postmenstrual days are used in order to correspond with the obstetrical literature.)

    Central Nervous System
    By 6 weeks the neural tube has closed. By 7 weeks the telencephalon, with a central and two lateral portions (the future cerebral hemispheres), and the diencephalon, from which the optic vesicles develop, have formed. There is a narrow part between the mesencephalon (the future midbrain) and rhombencephalon, and the rhombencaphalon has also formed two parts: the metencephalon (the future pons and cerebellum) and the myelencephalon, separated by the pontine flexure.

    SonoAVC enables us to dramatically visualize the (relative) growth of the different vesicles and yields images previously unobtainable in a routine clinical setting. Although developed to calculate the volume of spheroid or ellipsoid follicles, the SonoAVC algorithm uses the number of colored voxels to calculate an estimated volume and should therefore be equally accurate when calculating the volume of smaller and irregularly shaped structures.

    The choroid plexus is more apparent at 8 weeks and clearly visible at 9 weeks, although the cerebral ventricles still fill less than half of the cranium on the axial section. From 10 weeks it should be possible to visualize the cerebellum in around 80% of fetuses, and in all fetuses by 11 weeks.

    Cardiovascular System
    At 8 weeks the primitiveatria and ventricles have formed, the primitive interventricular foramen is still patent. The atria are wider than the ventricles and a double aorta still remains.

    Gastro-intestinal Tract
    The physiological umbilical hernia is present from 7 to 11 weeks. At 8 weeks the stomach should be visible in 31% of fetuses, at 10 weeks in 76% and from 11 weeks in all fetuses.

    Urorenal Tract
    The permanent kidneys start forming at 7 weeks. From 11 weeks onwards it should be possible to see the kidneys and bladder in 80% or more of fetuses.

    Limbs
    The arm and leg buds start developing at 6 weeks. At 7 weeks paddle-like hand and foot plates are visible and at 8-9 weeks fingers and toes have developed. The lower limbs lag behind the upper limbs by a day or two. During the 9th week the upper limbs rotate laterally and the lower limbs medially.

    Face
    Between 6 and 10 weeks the face is formed from the prominences developing from the first branchial arch around the stomodeum. Most studies investigating the age at which structures become visible are more than a decade old. As a result of the enhanced capabilities of the latest high resolution transducers, together with modern 3D and 4D software, these studies need to be carried out again, and may well result in some embryological data having to be rewritten. It is likely that today's ultrasound technologies are able to further reduce the current "no man's land" between pre -implantation and prenatal diagnosis.

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