[Author's note: This chapter was composed in 1993-95, and was an attempt to compile as complete as list as possible (to my meagre resources - this was before the Internet took off!) of fetal abnormalities that were thought up to that time to be associated with Trisomy 18, and to do a mini-review of each of them. It is not a list and discussion of all detectable abnormalities, and specifically not of all those associated with Downs Syndrome. Opinions of the relevance of each of the abnormalities below vary from "expert" to "expert" (which is exactly to the point!) and some authors may have have changed those opinions since they published articles I quote them in here. While the articles presented here may or may not have become too outdated for practical relevance, they remain of interest because they were written during the formative years of our fetal ultrasound paradigms. For that reason alone some of the abnormalities mentioned may be in need of as thorough an analysis as I have attempted on Choroid Plexus Cysts. For those fellow non-experts trying to make sense of our scan findings, and for parents trying to make sense of the findings on their own baby's scans, please make allowances for these factors. PL Ramm. March, 2004 ]
In this chapter, the fetal abnormalities that have been associated with T18 are listed and discussed with frequent quotation of statistics from the more recent and crucial articles. As it will become evident from reading these discussions, similar controversies to that of CPC are rife in the sonographic community. I have attempted to give both sides of each argument, when there is such a controversy.
“Soft” signs remain low risk in isolation but when combined with other abnormalities, the risk increases dramatically. It is worth remembering that chromosomal abnormalities are present in only 11% of malformed fetuses. Halliday93 recently reported than in Victoria in 1991, 50% of T18, 42% of T13, and 9.5% of T21 fetuses were identified by ultrasound examination in women less than 37 years of age (who therefore did not have routine amniocentesis.) Awareness of the following “soft” signs should increase these detection rates.
Several textbooks and review articles have in the early 1990’s detailed the common ultrasound findings one could expect with T18, particularly those by Sanders167, Nicolaides133, Eydoux73, Claussen51, Hegge99, Callen38 and Nyberg139, and these have been used extensively in the compilation of this section.
The cephalic index is the ratio of the bi-parietal diameter (BPD) to the occipito-frontal diameter (OFD) and provides an estimation of head shape. Down syndrome presents with a phenotypic brachycephaly or short OFD108. Theoretically, this would produce an increased cephalic index35. However the range of normal CI overlaps that seen in most cases of Down syndrome, so as an isolated finding this is not sensitive. Nicolaides133 found an association of 38%, spread over the common aneuploidies, but in fact most were seen in fetuses with T18 (19/53 cases).
According to Smith’s108, a prominent occiput and a narrow bi-frontal diameter are seen in 50% or more of T18 infants. Nicolaides133 showed the sonographic equivalent of this sign on the BPD view in 43/83 of fetuses with T18 and 1/42 with triploidy, termed it “Strawberry head” and gave an association with aneuploidy of 82%. Nyberg136 recognised it in 4/29 T18 fetuses between 14 and 24 weeks. This is not a widely reported finding and it may need to be highlighted to sonographers. In Nicolaides132 series, 10 euploid fetuses with this sign had other abnormalities and only two were born alive. Other syndromes, such as Robert’s syndrome may have this head shape.
Although Smith’s108 suggests less than 10% of T18 fetuses have cerebellar hypoplasia, Thurmond186 found all five of her T18 affected fetuses were smaller than expected on measurement of the transverse cerebellar diameter; Hill103 found 11/19. There is controversy as to whether IUGR (also seen in T18) affects cerebellar dimension; Reece158 saying it does not, Hill104 saying it does. The question arises: is a small cerebellum intrinsic to T18 or a result of the associated growth retardation? Partial agenesis of the vermis may be part of a Dandy-Walker variant, and as such is associated with T18, T13 and T21139.
Enlargement of the antero-posterior diameter of the cisterna magna is presumably related to the cerebellar dysgenesis associated with the Dandy-Walker variant. A posterior fossa cyst, representing an enlarged fourth ventricle, of the Dandy-Walker malformation has also been associated with T18. It was noted by Thurmond186 in her five cases (all in the third trimester.) Russ164 found two cases of T18 in 15 with the syndrome. Estroff71 found Dandy-Walker variant in 17 fetuses, five being aneuploid, two of which were T18. Nyberg137 found 9 cases of enlarged cisterna magna in 47 fetuses with T18, although eight of them had gestational age greater than 24 weeks. Nicolaides133 reported a posterior fossa cyst in 21 cases – eight being T18, six were T13, three were triploidy, and four were unspecified – but it is impossible to tell if he is referring to the Dandy-Walker malformation or merely to cisternal enlargement. Cisternal enlargement certainly appears to be more evident in the third trimester aneuploid fetus.
The absence of the cisterna magna and a deformed cerebellum (“banana” cerebellum) (Fig 17) are signs of the Arnold-Chiari Type II malformation which indicates an open defect of the spinal cord as found in meningomyeloceles134, 139, (Fig 16). Open spinal defects are occasionally associated with aneuploidy, but they have been reported in several series. Babcook6 found that 9/52 karyotyped fetuses with myelomeningocele were aneuploid (5 T18, 2 T13, 1 triploid, 1 translocation) and suggested cytogenetic analysis whenever this anomaly is found . Nyberg136 found 8/47 spinal defects in fetuses with T18. According to Sanders167, the risk of recurrence of spina bifida is lower if there has been a chromosomal association.
Fig 16: Sacral spina bifida cystica
Fig 17: "Banana" cerebellum and absent cisterna magna.
Often associated with other midline facial and cerebral abnormalities, such as the holoprosencephaly sequence (hence T13 and T18) and the Dandy-Walker malformation (hence T18), ACC as an isolated finding is unlikely to be detected earlier than the third trimester according to Vergagni191. Nicolaides136 series showed three T18 fetuses to have holoprosencephaly, ergo ACC. Two of Nyberg’s 47 T18 fetuses had ACC but they were both missed by ultrasound. A variety of other abnormal syndromes may be associated as well although as an isolated finding it may be a normal variant139. Due to the difficulty of early diagnosis of ACC with ultrasound, exact data are unknown.
Fluid accumulations around the fetal neck have been associated with chromosomal abnormalities, particularly Turner’s syndrome, when diagnosed in the second trimester139. Recently the use of first trimester transvaginal scanning has increased the range of findings in the fetal neck187,175. Just what the authors mean when they say “cystic hygroma” is often difficult to ascertain. The classic findings of cystica hygromata colli, in which relatively small, circumscribed cystic spaces are present antero-laterally in the neck at the region of the jugular lymphatic plexus, are rarely seen on the 18-20 week ultrasound, but are seen with greater frequency in the first trimester190.
In second trimester scans, the term is therefore more frequently used to describe the extensive posteriorly located effects of lymphangiectasia (lymphatic vessel dysplasia). This probably represents early hydropic change and it may extend to the scalp and the posterior thorax. It may be associated with generalized hydrops, effusions and ascites. The presence of septations in the cystic posterior cervical swelling is strongly indicative of chromosomal abnormality31. Brohnshtein29 found aneuploidy in 18/25 (72%) of septated posterior lesions as opposed to 6/106 (5.7%) of non-septated antero-lateral lesions. Monosomy X, Turner’s syndrome and T21 are the predominant chromosomal abnormalities. However, 10/47 of Nyberg’s136 T18 fetuses had “cystic hygroma.” Euploid conditions which involve lymphatic dysgenesis, such as Noonan’s syndrome, are missed at karyotyping.
Typically identified in the late first trimester, the presence of a translucent or oedematous region, as distinct from a cystic loculation, in the subcutaneous tissues at the posterior midline of the neck and upper thorax of the fetus has been strongly associated with aneuploidy, particularly T21 and T18, with the incidence of trisomic fetuses ranging from 19% to 88%. In his general series Nicolaides133 found nuchal oedema in 5/83 cases of T18. In another large study by Nicolaides130 8/33 aneuploid fetuses with a measurement over 3mm were T18 (total of 88 fetuses). In an elegant autopsy study, Hyett239 demonstrated cardiac defects in 56% of T21 fetuses with nuchal translucency and suggested that resultant early cardiac failure may be the cause for the finding rather than lymphatic problems. The almost universal association of cardiac defects with T18 would point to this as the cause here also. Pandya149 has suggested a measurement of ³3mm as a suitable cut-off for increased risk for T21. Nicolaides130 has suggested a 2.5mm measurement be used as the basis for a first-trimester screening protocol for T21. Roberts227 however found the sign too insensitive in a low-risk population to be useful for screening for Downs.
Fig 18: Nuchal translucency in a T21 fetus.
Hypotelorism (narrow-set eyes) to a variable degree (as far as cyclopia) is associated with the holoprosencephaly sequence (hence T13 and T18) and other midline defects. The narrow bi-frontal diameter in T18 may potentially present as hypotelorism in the second trimester. Nyberg136 noted it in 3/18 cases of T18, all in the third trimester. We have seen hypotelorism in T21 and T13 in the second trimester. Hypertelorism (wide-set eyes) is seen in several abnormal syndromes and in fetuses with frontal encephaloceles108.
Centrally cleft lip and palate have a high association with chromosomal abnormalities, particularly T13 and T18166. According to Sanders167, lateral or bilateral clefts are the more commonly seen, but have less association with aneuploidy. Most authors are not specific as to the nature of the cleft (lateral, central, bilateral) which it makes it difficult to draw an informed conclusion. Benacerraf10 found facial clefts in 5/9 T13 fetuses. Nyberg136 found clefts in 2/47 with T18. Nicolaides133 found them in 10/83 with T18 and 15/31 with T13 and only 1/69 with T21.
Fig 19: >Unilateral cleft lip.
Identified on the midline face profile view (Fig.20), hypoplasia of the mandible is noted in several aneuploidies, particularly T13 and T1810,16, and is generally associated with a poor prognosis. Bromley27, in a study of 20 cases of micrognathia, found T18 in 3, T13 in 1 and T9 in 1. Sixteen of these cases died or were terminated. Nyberg’s136 series of T18 fetuses described only 2/47, while Twining’s188 study found 5/7. Nicolaides133 found chromosomal abnormalities in 37/56 fetuses with this defect. In his series, 21/83 with T18 had micrognathia, 3/31 with T13, 9/42 with triploidy and 4 other aneuploidies.
Fig 20: Facial profile showing moderate degree of micrognathia (receding chin) in a T18 fetus.
The normal appearances of fetal ears were first described by Birnholtz21. He suggested a possible use of length measurement in predicting aneuploidy, due to the common association of ear anomalies affecting size, particularly in Down syndrome. Awwad5 found 3/4 T21 fetuses and 5/6 T18 fetuses to have small ears (below the 20th centile). Birnoltz21 found small ears (1.5 SD or more below the mean) in 2/6 T21, 3/3 T13, 3/3 T18 and 1/1 triploidy. Those with T18, T13 and triploidy showed the greatest deviation from the mean. Lettieri119 found 7/10 T21, 1/1 T18, 1/2 T13, and 1/1 triploidy fetuses to have small ears. This sign is being looked at with considerable interest, as it is relatively easy to perform. Gill228 found that while statistically significant differences from the normal were found in 25 T21fetuses, the results were not clinically useful in individual cases due to the wide range of normal ear lengths in his study.
Most, if not all T18 babies have a cardiac defect, usually an atrioventricular or ventricular septal defect108. However the ultrasound detection of such defects is not notably successful according to the literature. Stoll180, in a large survey performed in Strasburg, found that less than 1.3% of isolated VSDs were detected, but that this figure rose to 23% when there were other abnormalities. Equipment quality and sonographer training have significantly improved since the 1980’s when Stoll’s survey was carried out. DeVore64 used detailed grey-scale and careful colour Doppler techniques in identifying aneuploid fetuses in mothers who refuse or are unable to have invasive prenatal diagnosis with a success of 13/15 (87%) for T21 and 7/7 for T18. Copel57 found 11 (32%) cardiac defects in 34 aneuploid fetuses. Conversely, Paladini148 found 15 (48%) aneuploid fetuses in 31 cases of congenital heart disease. Wladimiroff198 diagnosed cardiac defects (VSD or DORV) in all 23 fetuses with either T13 or T18. All 7 of Twinings188 T18 fetuses had VSDs. Nyberg140 found 14 septal defects (either AV canal defects or VSDs) in 94 fetuses with T21, although only five (35%) were detected by ultrasound. In his series of 47 T18 fetuses, 29 had cardiac defects of unspecified nature, 18 (41%) of which were detected by ultrasound136. Nicolaides133 showed that 66% of fetuses with heart defect were aneuploid. About 1/3 of those with T21 and T18, and one half of those with T13 had cardiac defects demonstrated.
Careful scanning is required, using techniques which optimize the image in both grey-scale and colour Doppler. Other cardiac defects noted in T18 are double outlet right ventricle (DORV) and pulmonary or aortic stenosis. Twining188 reported echogenic foci within the ventricles in eight of 24 fetuses with choroid plexus cysts. Two of these had T18, one had T13, and another Turners. Petrikovsky225 and others have found these foci to be a normal finding, however Bromley224 noted an increased risk for T21. We have seen foci in a heart of a fetus with a VSD and other vascular anomalies, but with normal chromosomes.
Fig 21: VSD and echogenic foci in fetal heart. (Normal fetus)
A wide of renal anomalies have been associated with chromosomal abnormalities, from dysplasia and agenesis to mild pyelectasis. In T18, the commonly cited anomalies are horseshoe kidney (unlikely to be detected by ultrasound), cystic and dysplastic kidneys, unilateral agenesis, and a distended and non-visualized bladder. Nicolaides133 found renal defects in 25/83 (30%) of cases of T18. Nyberg136 found renal defects in seven of his 47 cases (15%). In Nyberg’s series the bladder was not visualized in two cases (with oligohydramnios in one case, but this may be related to the concurrence of myelomeningocele) and overly distended in another. Mild hydronephrosis has been touted as a fairly strong marker for T21 by Benacerraf15, who uses it in her and Nadel’s124, 17 scoring system for trisomy. In one series 3.3% of fetuses with pyelectasis had T21. Corteville58 also found an association, but only 17% of those with T21 had pyelectasis that they considered too low to warrant its inclusion in a screening test.
Fig 22: Renal dysplastic change (echogenic, cystic kidney)
During embryogenesis, a loop of midgut herniates into the base of the umbilical cord, rotates 90º counter-clockwise, then in the 12th week (menstrual age), retracts into the abdomen and rotates a further 180º. Omphalocele may be caused by failure of this retraction, or by failure of the midline ventral wall to close resulting in a secondary herniation of the gut as well as liver or other organs118. The association with T18 and T13 is high, particularly early in pregnancy, according to Snijders223. Her extensive literature review demonstrated chromosomal abnormalities in 149 (31%) of 474 cases. 89 (60%) of the aneuploid cases were T18. In Nicolaides’133 series, 42/116 fetuses with exomphalos were aneuploid, with 32/83 of the T18 fetuses having this condition. Gilbert86 found 17 cases of T18 in 35 cases of omphalocele.
Nyberg135 found aneuploidy in 8/8 cases without liver in the herniation and 2/18 with liver in the herniation. Getachew84 also noted that 4/6 fetuses with gut only in the omphalocele were aneuploid compared to 1/16 when liver was also present. DeVeciana63 noted absence of liver in the defect, small herniation size and the presence of other abnormalities to be highly suggestive of T18.
Fig 23: Omphalocele containing gut only. (Triploid fetus)
Most congenital diaphragmatic hernias (CDH) are of the Bochdalek type (posterior left defect), which causes pulmonary hypoplasia and is more usually lethal. Anterior hernias of Morgagni are less frequent. Structural or chromosomal abnormalities occur in over 50% of cases according to Nyberg139. Benacerraf16 found three cases in 15 fetuses with T18. Comstock52 found one case of T18 in eight CDH. In a large review, Bollman23 found that 6/33 fetuses were aneuploid, especially T18. In Nyberg’s136 series, 2/47 T18 fetuses had CDH. In Nicolaides’133 series, 17/41 fetuses with CDH were aneuploid, with 10/83 of the T18 fetuses having CDH.
Fig 24: Diaphragmatic hernia. Note stomach bubble next to displaced heart in the thorax.
A non-visualized stomach relates to oesophageal atresia115, a frequent malformation in T18. Nyberg136 and Bundy33 have noted this finding. More often than not, however, a tracheo-oesophageal fistula is present, and fluid can enter the stomach, which may be of normal size, or slightly smaller, particularly in the 2nd trimester. It may be that increased abdominal pressure in the 3rd trimester does not allow the filling of the stomach, making this finding more frequent in later pregnancy, where it is more likely to be related to hydramnios139 (Fig 25). A pouch of distended oesophagus may be seen in the upper thorax (Fig 26). 17/23 cases of OA were found to have T18 in Nicolaides’133 series.
Fig 25: Absent (small) stomach with polyhydramnios, suggestive of OA .
Fig 26: Distended pouch
in upper oesophagus in fetus with OA
(same as patient as Fig 25.)
(same as patient as Fig 25.)
Abnormal extremities, with cardiac defects, are the most common structural anomalies in T18, yet they are the most poorly identified with ultrasound. In Nicolaides’133 series, 43% with abnormal extremities were aneuploid, including 71/83 T18 fetuses. Nyberg136 detected only 50% of extremity abnormalities. Careful and methodical scanning is required to assess the extremities. Sonographers should familiarize themselves extensively with normal anatomy in order to better recognize deviations from the normal and improve detection in these areas.
Dramatically shortened or absent radius with or without shortened or absent thumb (radial ray syndrome) is seen in 10 - 50% of T18 cases. In Nyberg’s136 series four cases of radial aplasia or radial ray syndrome were missed by ultrasound. Sepulveda170 detected three cases with T18. Benacerraf10 noted two radial ray anomalies on 26 fetuses with T18. Twining188 saw this in one of his seven cases of T18. The hand is severely turned towards the radial side of the forearm (Fig 27). This is also seen in a variety of syndromes such as Robert’s Syndrome or Holt-Oram Syndrome.
Fig 27: Radial deviation of the hand in a fetus with Holt-Oram Syndrome
Anomalies of the hands are particularly common in T18. Jones108 states that over 50% have fixed clenched hands and a tendency for overlapping fingers with the index finger over the third finger and the fifth over the fourth being the usual configuration. In T18 these fingers are firmly clenched (camptodactyly) and cannot be extended. Extension movement of the fingers therefore excludes this sign. About 10% of cases may have extra fingers (polydactyly). In Nyberg’s136 T18 series 20/47 had clenched hands. In Benacerraf’s T18 series, 10/ 26 had fisted or clubbed hands. In Twining’s188 series of seven T18 cases, five had overlapping fingers, though two only were detected with ultrasound. Polydactyly is a very rare pick-up, but it is also seen in T13 fetuses. Carlson43 found hand postural deformities, hydramnios and other abnormalities to be indicative of aneuploidy.
Fig 28: Overlapping fingers in a fetus with unbalanced translocation T14.
The most frequent form of talipes is equinovarus, with plantar flexion and inversion of the foot. The toes are visible in a row on the same view which shows the long axis of the tibia, as in Fig:24. Rockerbottom foot is a posterior subluxation form of talipes (convex pes valgus, or vertical talus). Viewed from posteriorly, the heel is prominent as shown in Fig:25. This may be present in 10 -50% of T18 fetuses108. Nyberg136 detected only 1/5 cases with talipes in his series. 10/26 in Benacerraf’s T18 series had talipes. In Twining’s188 series of seven T18 cases, three had rockerbottom feet or prominent heels. Jones108 described a short, dorsi-flexed big toe in >50% of cases of T18. Sturla Eik-Nes showed such a case at ASUM in 1995† .
Fig 29: Talipes equinovarus (normal fetus)
Fig 30: Prominent heel in rockerbottom foot anomaly (triploid fetus)
Amniotic fluid (AF) disorders have been reported in many cases of T18. Oligohydramnios has been reported, but usually in association with renal dysplasia136. More commonly there is polyhydramnios. Damato61 found chromosomal abnormalities in seven out of 105 cases (6.7%) of increased AF, one each of T18, T13, a 4p deletion, and four cases of T21. Brady25 found four chromosomal abnormalities in 125 cases (3.2%) of idiopathic hydramnios: two were T18 (with IUGR) and two were T21. The paradoxical combination of IUGR and increased liquor has been noted by Nyberg136 (10/47), Eydoux73 (7/33) and Claussen51 (2/25). Carlson43, as mentioned, noted that the combination of hydramnios, hand posturing abnormality and any another anomalous finding was consistent in 6 trisomic fetuses (27%) of 49 cases of hydramnios; three with T18, two with T21 and one with T13.
A high association of growth retardation with T18 has been noted; in particular, an association with symmetrical IUGR has been proposed. 27% of fetuses were “growth retarded” or small for expected gestational age in the surveyed literature, although in some individual reports the frequency is 50% or greater (Nicolaides133 48/83, Thorpe-Beeston185 14/17). This effect is particularly noticeable in those cases which were diagnosed in the 3rd trimester, although Nyberg136 found a smaller than expected fetus in eight of 29 cases scanned at 21 weeks or less. IUGR with increased or normal liquor is also an unexpected and common finding in all chromosomal abnormalities. Absent or reversed end-diastolic flow in the umbilical artery has a very high correlation with IUGR. Rizzo160 evaluated the karyotype of 192 fetuses with absent end-diastolic flow on umbilical artery Doppler, known to be associated with placental failure, severe growth restriction and poor fetal outcome. 16 (8.3%) of these fetuses were aneuploid; nine T18, four T21 and one 14p deletion. The supposed association of symmetrical growth retardation was not evident in Rizzo’s study, nor in Snijders176, where HC/AC ratios were also increased without any significant difference from the HC/AC ratios in their chromosomally normal but growth retarded fetuses. Snijders176 found no increase in Doppler abnormalities to explain her findings however.
Parilla229 studied 50 cases of isolated SUA and found none with chromosomal abnormality. Nyberg136 however found it in 9/47 T18 fetuses. Saller165 found six SUA in 53 aneuploid fetuses; two were T18, two were T13, and the other two were unusual translocations. In another study Nyberg138 found that 12/30 fetuses with SUA had other major abnormalities and six of these were aneuploid. Hence the presence of other abnormalities is crucial in determining the significance of this finding (similar to CPC). There is an association with IUGR and post-natal renal reflux noted by Sepulveda169. Detection is quite difficult in obese patients, and Jones109 suggests that only 65% are detected overall. Using colour flow to assess blood-flow in the umbilical arteries around both sides of the bladder is probably the best diagnostic technique when clear images of the cord in the amniotic fluid cannot be obtained. Jeanty107 noted that the lower abdominal aorta at the bifurcation curves way from the side on which the artery is absent, and that the iliac artery is smaller.
Fig 31: Single umbilical artery (otherwise normal fetus).
Nyberg136 saw 2 cases of allantoic cysts (remnants
of the funicular diverticulum of the urogenital sinus) in his T18 series,
but these were associated with omphaloceles, which corresponds with the earlier
findings of Fink76. It
may be that their association is with omphalocele rather than with chromosomal
abnormality independently. Jauniaux230
reported on two cases of “pseudocysts” in the cords of T18 fetuses.
Hegge99 described multiple cord cysts in a case of T18.
Cystic degeneration of Wharton's jelly is not infrequent in normal fetuses.
Cystic degeneration of Wharton's jelly is not infrequent in normal fetuses.
The abnormalities discussed above while not specific to T18 are commonly associated with it. The following abnormalities have also been detected on ultrasound in association with chromosomal abnormalities, particularly T21. They are referenced to the bibliography, but not discussed in detail.
Shortened Fronto-thalamic Distance.7
Thickened Nuchal Fold (>6mm at 18 weeks) 9, 11, 151
Open Mouth, Protruding Tongue. 99,133
Renal Dilatation. 58,15,17,18,124,220
Duodenal Atresia. 99,38,133,140,
Echogenic Bowel. 140,220,124,168,28
Ascites / Hydrops. 133,140
Lengthened Iliac Wing Distance. 1
Shortened Humerus. 162,32,19,161
Hitchhiker’s Thumb. 43,
Hypoplasia 5th finger, Middle Phalanx. 13,20
Shortened 5th finger. 13,20
Inward Curving 5th Finger (Clinodactyly). 99
Shortened Femur. 220,17,18,
Wide-spaced 1st and 2nd toes (Sandal Gap). 167
These soft signs complete the list devised for the Chromosomal Pro-Forma discussed in Chapter 15.
The search for other abnormalities is the crucial part of an ultrasound examination of a fetus with CPC. While all areas should be examined in detail, looking for the markers discussed above, it is by the careful evaluation of the cranial shape and contents, and the face, correct scanning of the heart and detailed assessment of the extremities that the subtle yet common findings in T18 will be uncovered. Some findings are more easily detected or more prominent in the third trimester. Examples of such findings are IUGR, agenesis of the corpus callosum, amniotic fluid disorders, cisterna magna enlargement and cardiac defects. Attention to technique and a use of good machine can only allow the anatomy to be displayed on the screen. In order for this to be of any use, the sonographer must firstly recognize abnormalities. It must be stressed that detailed scanning of normal fetuses during the 18 - 20 weeks scan is strongly recommended, within the constraints of available time and image quality, so that sonographers become extremely familiar with fetal structures in their normal anatomical configurations. Only when knowledge, experience and scanning skill are united in the well-trained sonographer will most abnormalities be recognized, and a presumptive diagnosis of chromosomal abnormality suggested, guiding the management of these at-risk pregnancies.
When abnormal findings are seen to place the fetus in this at-risk category, genetic counseling and analysis of risk may indicate the need for amniocentesis. These matters are discussed fully in the following chapters.
† Eik-Nes, S.: ASUM Annual Scientific Meeting, Sydney, 1995.