The 18+0–20+6 Weeks Fetal Anomaly Scan National Standards

Ultrasound 2010;18:92-98
doi:10.1258/ult.2010.010014
© 2010 British Medical Ultrasound Society

 

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Education and Service Delivery


Trish Chudleigh 


Rosie Ultrasound Department, Cambridge University Hospitals NHS Foundation Trust, Hills Road, Cambridge CB2 0QQ, UK

Email: trish.chudleigh{at}addenbrookes.nhs.uk




Abstract

TOP

Abstract
The gestational age window

Appointment timing and frequency

The focus of the…

Markers, normal variants and…

The base menu

The base menu in…

The implications of normal…

The meaning of normal…

Conclusions

References

 

Ultrasound screening for fetal abnormalities has been availableto pregnant women in England for well over two decades. Historically,clinical departments have used guidance provided by the RoyalCollege of Obstetricians and Gynaecologists to inform them asto what should be included for assessment during the examination.Recently, the Department of Health commissioned the Fetal AnomalyScreening Programme to develop and extend the second trimesteranomaly scan to ensure an effective and accessible service forall pregnant women in England. The Fetal Anomaly Scan NationalStandards have now been launched and will impact on all departmentsproviding this service. This paper summarizes the backgroundto this work, identifies the subtle shift of focus requiredin the new anomaly scan and explains the rationale behind thesechanges.

Keywords: Ultrasound, antenatal, second trimester, standards, abnormality


Second trimester ultrasound screening for fetal anomalies has been offered routinely to all pregnant women in England since the mid-1980s, with the majority of departments using the protocol and standards published by the Royal College of Obstetricians and Gynaecologists (RCOG) in 2000 as the basis for their individual screening programmes.1

The responsibility for developing a screening programme thatwas uniform in its scope and in its effectiveness, and was accessibleto all pregnant women in England, was given to the NHS FetalAnomaly Screening Programme (FASP) by the Department of Healthin 2004. This led, in January 2010, to the launch of a nationalset of standards for the first time. The document ‘The18 to 20 Weeks Fetal Anomaly Scan National Standards and Guidancefor England’ describes 12 Standards. The start date forimplementation across all Trusts in England has been set forApril 2011.

The Standards support the principal objectives of FASP which are to:

  • Ensure access to a uniform screening programme which confirms to an agreed level of quality;
  • Provide appropriate information for women so that they can make an informed choice;
  • Offer choices to women about their screening options and pregnancy management;
  • Identify serious fetal abnormalities, either incompatible with life or associated with morbidity, allowing women to make reproductive choices;
  • Identify certain abnormalities that may benefit from antenatal intervention; and
  • Identify certain abnormalities that require early intervention following delivery.2

The document was prepared by a multidisciplinary group ofexperts, convened by FASP in 2006. The group’s remit was toprepare a set of guidelines which would underpin a service that,first, women would want to access, second, that sonographerscould realistically provide and, third, that could be auditedand monitored locally and nationally. This work was then takenforward by the National Ultrasound Screening Standards CoreReference Group, established in 2008.

The Standards relate to an ultrasound examination provided between18 weeks 0 days and 20 weeks and 6 days to pregnant women whohave either already undertaken screening for Down’s syndromeand have a low-risk result (i.e. less than 1 in 150), or havedeclined Down’s syndrome screening.

In addition to addressing the content of the scan itself, theStandards address a wide range of issues relating to the routineanomaly service including access, governance, leadership andservice provision, clinical arrangements, prescan preparation,image capture, storage and archiving, performance evaluation,training and professional competence, ultrasound safety, purchasingequipment and the independent sector.





The gestational age window

TOP

Abstract

The gestational age window
Appointment timing and frequency

The focus of the…

Markers, normal variants and…

The base menu

The base menu in…

The implications of normal…

The meaning of normal…

Conclusions

References

 

The Standards recommend that the routine anomaly scan should be performed between 18 weeks 0 days and 20 weeks and 6 days. The lower limit of the gestational age window was selected to acknowledge the ability to examine the fetal anatomy thoroughly and competently from 17 completed weeks (i.e. 18+0 weeks). The upper limit of 20+6 weeks was selected to allow sufficient time for referral, the subsequent ultrasound examination to be undertaken and, when indicated, termination of pregnancy to be performed before 21+6 days. This is because feticide is always recommended for the termination of pregnancy performed after this gestation. The Standards (Standard 4 – Clinical arrangements) recommend that a woman with a suspected or confirmed fetal anomaly should be seen by an obstetric ultrasound specialist within three working days of the referral being made or by a fetal medicine unit within five working days of the referral being made.2





Appointment timing and frequency

TOP

Abstract

The gestational age window

Appointment timing and frequency
The focus of the…

Markers, normal variants and…

The base menu

The base menu in…

The implications of normal…

The meaning of normal…

Conclusions

References

 

The Standards (Standard 4 – Clinical arrangements) also recommend that the fetal anomaly scan appointment should be 30 minutes duration for a singleton pregnancy and 45 minutes for a multiple pregnancy. The appointment time should include ‘prescan counselling, the examination itself, postscan counselling and reporting’.2

Repetitive strain injuries are being experienced by an increasing number of sonographers, due in part to the high body mass index (BMI) of an increasing number of pregnant women. These factors, together with the implications of an under-staffed workforce, have been acknowledged within the Standards with the limiting of repeat examinations. Where the image quality of the scan is compromised by increased BMI, uterine fibroids, abdominal scarring or a suboptimal fetal position, the Standards recommend that a single further scan at 23 weeks should be offered to complete the screening examination. Where adequate assessment of the fetal anatomy remains compromised following the second scan, the woman should be told that her screening examination remains incomplete and this should be recorded in her report.2





The focus of the FASP routine anomaly scan

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Abstract

The gestational age window

Appointment timing and frequency

The focus of the…
Markers, normal variants and…

The base menu

The base menu in…

The implications of normal…

The meaning of normal…

Conclusions

References

 

This article addresses principally Standard 6 (the 18+0 to 20+6 weeks fetal anomaly screening scan), the rationale of which states ‘The 18+0 to 20+6 weeks ‘ultrasound base menu’ has been devised to provide consistency in the ultrasound procedure in terms of specifying techniques to be used to obtain fetal measurements and defining what anatomical structures should be assessed by professionals during the examination’.2

The emphasis of the routine anomaly scan will shift subtly in departments where the FASP guidelines supersede those of the RCOG. This is because the focus of the FASP anomaly scan is to screen only for 11 specific conditions. The conditions, together with their required detection rates, are listed in Table 1. These conditions were selected because they have a significantimpact on the likely outcome of the pregnancy being screened.Some, such as anencephaly or bilateral renal agenesis, are incompatiblewith life; others, such as spina bifida or serious cardiac abnormalities,are associated with significant morbidity and others, such asgastroschisis or cleft lip, benefit from immediate postnatalsupport. In addition, review of the published literature overthe last 20 years indicates that the prenatal ultrasound detectionrate of all 11 conditions exceeds 50%.



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Table 1 Eleven conditions screened for by the routine anomaly scan

 

One of the underlying aims of the programme is to improve the detection rate of serious cardiac abnormalities. A survey of ultrasound units in 2002 indicated that 97.5% of units included the four-chamber view of the heart in their routine anomaly scan and 56.9% also included the outflow tracts.3 While it is acknowledged that detection rates from current screening programmes are likely to be higher than the 23.4% (range 3–63%) rate reported by Bull for serious congenital heart disease from cardiac screening programmes in 1994, this is currently the largest data-set available from UK screening programmes.4 The programme has, therefore, set an initial target detection rate for serious cardiac abnormalities of 50%. It acknowledges that a training programme must be implemented to ensure that all sonographers in England have the skills to evaluate the fetal heart properly, using the views set down in the Standards’ Fetal Cardiac Protocol.2

A further aim of the programme is to improve the prenatal detection of cleft lip. The 2002 survey indicated that 81.7% of units included assessment of the lips in their routine anomaly scan.3 These data can be compared with the prenatal detection rate of facial clefts within the UK, which varies from 17.5% to 75%.5 The programme has set an initial target detection rate forcleft lip of 75%. To achieve this target a component of theFASP routine anomaly scan is a coronal view of the lips andnose.

The FASP routine anomaly scan contains no requirement to assessthe fetal profile, the long bones other than one femur, thecarrying angles of the feet and hands or examination of thefingers and toes. This was a conscious decision arising fromthe purpose of the examination. A competent evaluation of theanatomical requirements of the programme should enable eachof the 11 conditions to be excluded or suspected and diagnosed(to the agreed detection rate). The benefit which examiningthe anatomy listed above conferred to the detection of the 11conditions in every pregnancy was not considered sufficientto justify the time required for their competent evaluation.





Markers, normal variants and ultrasound appearances

TOP

Abstract

The gestational age window

Appointment timing and frequency

The focus of the…

Markers, normal variants and…
The base menu

The base menu in…

The implications of normal…

The meaning of normal…

Conclusions

References

 

The second significant change in practice that the new Standards will initiate relates to markers for abnormal karyotype. Since they were first reported, the sensitivity of specific ultrasound appearances in screening for the more common aneuploidies has been controversial. The sonographic markers for aneuploidy described in the RCOG document have been incorporated into the protocols and guidelines in many departments. These ultrasound features are listed in Table 2 and include what many practitionerswould now perhaps consider to be a combination of structuralabnormalities (e.g. ventriculomegaly, cleft lip, talipes) and‘conventional’ soft markers (e.g. choroid plexuscysts, sandal gap, clinodactyly).



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Table 2 Sonographic markers for abnormal karyotype (adapted from the RCOG2)

 

The evidence to support a significant association between the ever varying list of ultrasound appearances described, with equally varying sensitivity, as ‘soft markers’ is poor. This remains true in populations at increased risk of Down’s syndrome and also in the small number of studies reported from populations at low risk of Trisomy 21. Smith-Bindman’s meta-analysis of articles published between January 1980 and February 1999 described the second trimester sonographic markers reported to be associated with chromosomal abnormalities.6 The authors calculated estimates of sensitivity, specificity and likelihood ratios in the detection of Down’s syndrome for choroid plexus cysts, hyperechoic bowel, hyperechoic cardiac focus, femoral shortening, humeral shortening, renal pyelectasis (4.0 mm anteroposterior [AP]) and thickened nuchal fold (6.0 mm). The results were stratified depending on whether the markers were present as an isolated finding or were identified in association with fetal structural abnormalities. A total of 56 articles described 1930 fetuses with Trisomy 21 and 130,365 unaffected fetuses. Forty-nine (88%) of the studies included women at increased risk of abnormal karyotype based on age (n = 39), serum biochemistry (n = 36) or family history (n = 16).The authors concluded that increased nuchal fold was the only marker which may be useful in screening for Trisomy 21 and that the other markers, when present in the absence of associated structural abnormalities, could not discriminate well between fetuses unaffected by and those affected with Trisomy 21.6

These data were significant in supporting the programme’s rationale to screen only for 11 specific conditions and in pregnant women who had already undertaken screening for Down’s syndrome and whose pregnancies were reported as being at low risk for the condition. The Standards document incorporates the Best Practice Guidance, published as a Programme Statement in July 2009, on normal variant screening in pregnancy. The guidance, to take effect from 1 April 2010, recommends the following management for women who have either declined Down’s syndrome screening or whose nationally approved Down’s syndrome screening test identifies them as ‘low risk’ and in whose fetus one or more of four ultrasound appearances are present. These appearances are choroid plexus cysts, dilated cisterna magna, hyperechoic cardiac foci and two-vessel cord. Pregnancies in which these four appearances are identified during the anomaly scan, either as an isolated finding or together, should not be referred for further ultrasound assessment of chromosomal abnormality. These appearances, which should be considered as normal variants, are listed in Table 3.



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Table 3 Ultrasound appearances which should be considered as normal variants and which should not be referred for further assessment of chromosomal abnormality, whether present as an isolated finding or together

 

There are five other appearances, however, which should initiate referral for further assessment and these are listed in Table 4. Sonographers will be aware of the association between increasednuchal fold and Trisomy 21. It must be remembered that increasednuchal fold can also be associated with cardiac anomalies, infectionand genetic syndromes, and it is for this range of reasons thatincreased nuchal fold requires referral. In the absence of centralnervous system abnormality, ventriculomegaly is associated withfetal infection, intrauterine haemorrhage and a range of chromosomalabnormalities, including trisomies 13, 18 and 21 and triploidy.Hyperechoic bowel is associated with infection and cystic fibrosisor may be a reaction to fetal swallowing following intra-amnioticbleeding. Renal pelvic dilation requires third trimester surveillanceas a progressive increase in pelvic size is an indicator forpostnatal imaging in order to exclude urinary tract obstructionand/or reflux. Small size compared with measurements at datingmay be associated with placental insufficiency, genetic diseaseor infection or may be due to constitutional smallness. Thesefive appearances, therefore, require referral, principally forissues relating to perinatal management, rather than an associationwith abnormal karyotype.



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Table 4 Ultrasound appearances which should be referred for further assessment





The base menu

TOP

Abstract

The gestational age window

Appointment timing and frequency

The focus of the…

Markers, normal variants and…

The base menu
The base menu in…

The implications of normal…

The meaning of normal…

Conclusions

References

 

As stated above, the FASP Standards re-define the purpose ofthe anomaly scan and the sonographer’s awareness of this factis critical. It is, however, equally important that all health-careprofessionals with whom the woman interacts – prior toher making her informed decision to attend for her anomaly scan– fully understand the purpose, implications and limitationsof the FASP routine anomaly scan.

The routine anomaly scan will be audited against a base menu,which is the FASP equivalent to a ‘tick list’. Thebase menu divides the examination into seven sections and requiresassessment of 13 fetal anatomical areas, together with the subjectiveassessment of the amniotic fluid and reporting of placenta site.For the relevant part of the fetal anatomy to be evaluated as‘normal’, it must be normal in position, appearanceand size.

The fetal biometry required includes measurement of the head circumference (HC), abdominal circumference (AC) and femur length (FL). Assuming the ultrasound expected date of delivery (USEDD) has been assigned at the earlier dating or combined screening scan, fetal growth should be assessed from these measurements using Loughna et al.7 size charts. Where dating and assessment of the fetal anatomy is required, the HC and FL should be used to assign the USEDD using Altman and Loughna dating charts.7

Table 5 lists the main features of the ultrasound basemenu and provides examples of the six standard images to betaken and stored.



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Table 5 The 18+0–20+6 weeks fetal anomaly ultrasound scan base menu





The base menu in practice

TOP

Abstract

The gestational age window

Appointment timing and frequency

The focus of the…

Markers, normal variants and…

The base menu

The base menu in…
The implications of normal…

The meaning of normal…

Conclusions

References

 

It is important to remember that the base menu describes theroutine ultrasound examination that every pregnant woman inEngland should be offered and, therefore, the minimum ultrasoundstandard that women can expect. This standard does not restrictdepartments which may wish to provide a more extensive routineanomaly examination.

Sonographers must remember that, although they are now screeningfor 11 specific conditions, this does not (neither should it)prevent them from identifying, and referring on, appearancesother than those associated with the 11 conditions.

The programme is not, for example, screening for encephalocoeleor unilateral multicystic kidney. However, it is hoped thatany competent sonographer would both report such findings andrefer on appropriately if either were identified during theroutine anomaly scan, in just the same way as they do at present.

Similarly, the programme has no requirement to evaluate thecarrying angle of the feet or to assess the fingers and toes.Thus, if bilateral talipes or missing digits of one hand werenoted ‘in passing’, it is reasonable to expect thesonographer to report such findings and refer on appropriately.What is important for the sonographer, other health professionalsand the woman being screened to remember is that, because theseappearances are not being specifically screened for, it is incorrectfor a woman to consent to her routine anomaly scan with theexpectation that one or all of those appearances will be identifiedif present in her fetus.





The implications of normal findings

TOP

Abstract

The gestational age window

Appointment timing and frequency

The focus of the…

Markers, normal variants and…

The base menu

The base menu in…

The implications of normal…
The meaning of normal…

Conclusions

References

 

The implications of normal ultrasound appearances for each ofthe seven sections of the base menu are considered below.


Section 1 – head and neck

Assessment of the head and neck requires examination of theskull shape and appearance and examination of the intracranialanatomy in two sections.

Normal appearances of the skull should exclude 98% of casesof anencephaly, the ‘lemon sign’ associated withopen spina bifida and should contribute to the exclusion of60% of cases of lethal skeletal dysplasia (in particular achondrogenesis[type 1] and osteogenesis imperfecta [type 11a]). The rangeof skeletal dysplasia which is lethal is extensive and the ultrasoundappearances associated with the specific diagnoses are varied.Ultrasound findings that should raise the suspicion of a lethalskeletal dysplasia are most likely to arise from evaluationof the fetal skull, chest and femur. It should be rememberedthat the lethality of skeletal dysplasias is primarily the resultof pulmonary hypoplasia associated with a small chest.

The intracranial structures that should be assessed include the cavum septum pellucidum, the ventricles and the cerebellum. As the section described by Loughna et al. for measurement of HC is the ‘ventricles’ level, measurement of both HC and the posterior horn of the lateral ventricle are taken in the same section. An image demonstrating measurement of the HC and the posterior horn is required. Measurement (‘inner to inner’) of the posterior horn should be made by placing the callipers across the widest part of the lateral ventricle (at the level of the glomus of the choroid plexus) and perpendicular to the long axis of the ventricle, in accordance with the 2007 ISUOG Guideline.8

A suboccipitobregmatic view is required to assess the cerebellum and measure the transcerebellar diameter (TCD). The nuchal fold should also be assessed and measured if it looks subjectively increased. The nuchal fold measurement describes the distance between outer border of the occiput and the outer border of the skin.9,10 A nuchal fold measurement greater than 6.0 mmrequires referral. A suboccipitobregmatic image demonstratingmeasurement of the TCD is required.

Normal appearances of the intracranial anatomy at the ‘ventricles’level should exclude ventriculomegaly and contribute to theexclusion of 95% of cases of Trisomy 13.

Normal appearances of the cerebellum and nuchal fold shouldexclude the ‘banana’ sign associated with open spinabifida and should exclude a significant increase in Down’s risk,respectively.


Section 2 – face

Assessment of the face requires examination of the lips andnasal tip in coronal view only. This is because one of the conditionsbeing screened for is cleft lip. It should be remembered thatthe programme is not screening for cleft palate. There is norequirement to image the fetal profile or the orbits. This isbecause assessment of the fetal profile and/or orbits is ofminimal value in screening for the programme’s 11 conditions.

An image of the coronal view of the lips and nasal tip is required.

Normal appearances of the coronal lips and nasal tip view shouldexclude 75% of cleft lips and contribute to the exclusion of95% of cases of Trisomy 13.

Conversely, one-quarter of all cleft lips will not be excludedby these normal appearances, neither will any clefting of thealveolar ride or palate, nor an absent nasal bone, micrognathia,a flat forehead or frontal bossing.


Section 3 – chest

Assessment of the chest requires examination of the lungs andthe heart. The Fetal Cardiac Protocol involves four basic intracardiacviews, namely laterality, the four-chamber view, the left outflowtract and lastly the right ventricular outflow tract or, alternatively,the three-vessel view.

Normal appearances of the lungs and four basic cardiac viewsshould contribute to the exclusion of 60% of cases of diaphragmatichernias and should exclude 50% of serious cardiac abnormalities.Normal appearances of the chest and its normal proportions relativeto the abdomen should contribute to the exclusion of 60% oflethal skeletal dysplasias (e.g. achondrogenesis, thanatophoricdysplasia, short-ribbed polydactyly) and should contribute tothe exclusion of 95% of cases of Trisomy 18.

Conversely normal appearances will fail to identify 40% of diaphragmatichernias and will fail to identify half of all serious cardiacabnormalities.


Section 4 – abdomen

The anatomical structures that should be assessed within theabdomen include the stomach, the bowel, the cord insertion,both kidneys, both renal pelves and the fetal bladder. Referralshould be made if the echogenicity of the bowel is equivalentto that of bone. The renal pelves should be measured if theyappear subjectively large, with referral being made if the APdiameter of one or both is >7.0 mm. The fetal bladder mustbe seen in both sagittal and transverse views.

The AC should be measured at the level described by Loughna et al.7 An image demonstrating measurement of the AC is required.

Normal appearances of the abdomen should exclude 98% of casesof gastroschisis, 80% of cases of exomphalos, 84% of cases ofbilateral renal agenesis and contribute to the exclusion of60% of cases of diaphragmatic hernia, 95% of cases of Trisomy13 and 95% of cases of Trisomy 18.

Conversely, normal appearances of the abdomen will fail to identifyall cases of right-sided diaphragmatic hernia, one in five casesof omphalocoele and approximately one in six cases of bilateralrenal agenesis. In addition, no cases of two-vessel cord willbe identified.


Section 5 – spine

The spine and its skin covering should be assessed in both sagittaland transverse sections.

An image of the spine and its skin covering in sagittal sectionis required.

Normal appearances of the spine, its skin covering and the intracranialanatomy together with the shape of the skull should exclude90% of cases of open spina bifida.

Conversely, normal appearances of the spine, its skin coveringand the intracranial anatomy together with the shape of theskull will fail to identify one in 10 cases of open spina bifida.


Section 6 – limbs (a) and limbs (b)

Assessment of the limbs requires (a) measurement and evaluationof the appearance of one femur together with (b) the presenceof the metacarpals of both hands and the metatarsals of bothfeet. There is no requirement to evaluate the carrying angleof either foot or either hand. There is also no requirementto examine the four fingers and the thumb of either hand orthe five toes of either foot.

The FL should be measured as described by Loughna et al.7 Animage demonstrating measurement of the FL is required.

The normal appearance and length of the femur examined shouldcontribute to the exclusion of 60% of lethal skeletal dysplasias(e.g. achondrogenesis, campomelic dysplasia, thanatophoric dysplasias).

The normal appearances of the hands and feet should confirmthe presence of both hands and both feet.

Conversely, normal appearances of the hands and feet, as definedby the programme, will not confirm the presence of four fingersand one thumb on each hand or the presence of five toes on eachfoot. Such normal appearances, as defined by the programme,will not exclude fixed flexion deformities of the hands and/orthe presence of talipes.


Section 7 – uterine cavity

Assessment of the uterine cavity requires the subjective evaluationof the amniotic fluid volume. It also requires confirmationthat the placenta has been visualized and that its positionhas been noted.





The meaning of normal findings

TOP

Abstract

The gestational age window

Appointment timing and frequency

The focus of the…

Markers, normal variants and…

The base menu

The base menu in…

The implications of normal…

The meaning of normal…
Conclusions

References

 

The routine anomaly examination screens for 11 conditions. Theconverse of this aim is that there is a range of abnormalitiesthat are not screened for and, therefore, their detection isnot a component of the routine anomaly scan. It must be rememberedthat such abnormalities may be present, but a report of ‘normalfindings’ within the context of the FASP programme iscorrect.





Conclusions

TOP

Abstract

The gestational age window

Appointment timing and frequency

The focus of the…

Markers, normal variants and…

The base menu

The base menu in…

The implications of normal…

The meaning of normal…

Conclusions
References

 

The aims of the FASP Standards shift considerably the emphasis of the ‘traditional’ routine anomaly scan and this will impact not only on the responsibilities of sonographers but on those of all health-care professionals involved in the delivery of antenatal care. It is the responsibility of all these individuals to be aware of the purpose, implications and limitations of this new programme and to ensure that all pregnant women accepting FASP 18+0 to 20+6 weeks fetal anomaly screeningare also aware of, and fully understand, this range of information.

 

 

 

 

 




References

TOP

Abstract

The gestational age window

Appointment timing and frequency

The focus of the…

Markers, normal variants and…

The base menu

The base menu in…

The implications of normal…

The meaning of normal…

Conclusions

References

 

  1. Royal College of Obstetricians and Gynaecologists. Routine Ultrasound Screening in Pregnancy: Protocol, Standards and Training. London: RCOG Press, 2000
  2. NHS Fetal Anomaly Screening Programme. The 18 to 20 Weeks Fetal Anomaly Scan National Standards and Guidance for England. Exeter: NHS Fetal Anomaly Screening Programme, 2010
  3. UK National Screening Committee. Antenatal Ultrasound Screening Ultrasound Survey of England: 2002. Oxford: UK National Screening Committee Programmes Directorate, 2005
  4. Bull C. Current and potential impact of fetal diagnosis on prevalence and spectrum of serious congenital heart disease at term in the UK. British Paediatric Cardiac Association. Lancet 1999;354:1242–7[Medline]
  5. Chudleigh T, Cook K. The impact of training on the detection of facial clefts. Ultrasound 2004;12:196–200[Abstract/Free Full Text]
  6. Smith-Bindman R, Hosmer W, Feldstein VA, Deeks JJ, Goldberg JD. Second-trimester ultrasound to detect fetuses with Down Syndrome: a meta-analysis. JAMA 2001;285:1044–55[Abstract/Free Full Text]
  7. Loughna P, Chitty L, Evans T, Chudleigh T. Fetal size and dating: charts recommended for clinical obstetric practice. Ultrasound 2009;17:161–7
  8. ISUOG Guidelines. Sonographic examination of the fetal central nervous system: guidelines for performing the ‘basic examination’ and the ‘fetal neurosonogram’. Ultrasound Obstet Gynecol 2007;29:109–16[Medline]
  9. Benacerraf BF, Barss VA, Laboda LA. A sonographic sign for the detection in the second trimester of the fetus with Down’s syndrome. Am J Obstet Gynecol 1985;151:1078[Medline]
  10. Benacerraf BF, Frigoletto FD, Laboda LA. Sonographic diagnosis of Down syndrome in the second trimester. Am J Obstet Gynecol 1985;153:49[Medline]

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