A Role for Ultrasound: Earlier in the first Trimester.
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
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.
Copyright © Mednet Healthcare Ltd 2006. All rights reserved.