The implementation of first‐trimester scanning at 10–13 weeks' gestation and the measurement of fetal nuchal translucency thickness in two maternity units
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Abstract The aim of this prospective screening study was to evaluate the implementation of an additional ultrasound examination, incorporating the measurement of fetal nuchal translucency thickness, at 10–13 weeks' gestation in two maternity units providing routine antenatal care. During the 1 year prior to the introduction of the first‐trimester scan, the major indication for fetal karyotyping was maternal age ≥ 35 years and only two out of the total of 11 cases of trisomy 21 were identified. In the first 5 months of the study, 70% of the women delivering in these hospitals attended for measurement of fetal nuchal translucency thickness and the measurement was obtained in all cases. This was achieved without an increase in the number of sonographers or ultrasound machines. The incidence of fetal nuchal translucency thickness ≥ 2.5 mm was 3.6% (63 of 1763), and this group included three of the four fetuses with trisomy 21. The findings of this study demonstrate the feasibility of introducing scanning at 10–13 weeks' gestation and the measurement of fetal nuchal translucency thickness in routine maternity units. The sensitivity and specificity of this method of screening are at present being evaluated in a large multicenter study. Copyright © 1995 International Society of Ultrasound in Obstetrics and GynecologyKeywords:
Nuchal translucency
Nuchal Translucency Measurement
Second trimester
Abstract Objective To determine the visualization rates of fetal anatomic structures by three‐dimensional ultrasound (3DUS) at 12–13 weeks of gestation. Study Design This was a prospective observational study of women presenting for nuchal translucency ultrasound. Five 3D volumes of the fetus were acquired transabdominally. Two investigators independently reviewed the stored volumes offline following a standardized protocol. Results One hundred singleton fetuses were examined. The mean time for 3D volumes acquisition was 4.8 min; and for 3D review 17 min. Anatomic structures were seen as follows: cranium, lateral cerebral ventricles and abdominal wall 100%; stomach, vertebrae, upper and lower limbs ≥ 94%; face 71%, bladder 58%, both kidneys 39%, skin overlying spine 26% and heart 18%. Agreement between two observers ranged from 100% (for head, abdominal wall and lower limbs) to 43% (for visualization of skin overlying spine). A complete basic anatomic survey was achieved in 11.4% of the 12‐week fetuses and 33.3% of the 13‐week fetuses ( p ‐value = 0.038). Conclusions First‐trimester transabdominal 3DUS was adequate for assessment of the head, abdominal wall, stomach, limbs and vertebral alignment. It was less effective for evaluating the heart and intactness of the skin over the spine. Copyright © 2010 John Wiley & Sons, Ltd.
Nuchal translucency
Nuchal Translucency Measurement
Crown-rump length
Conjunction (astronomy)
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Objective: To examine the longitudinal course of nuchal translucency thickness by weekly measurements between 10 and 15 weeks' gestation in normal fetuses. Methods: Nuchal translucency was measured weekly from 10 to 15 weeks' gestation in 64 fetuses with normal pregnancy outcome. The median and the fifth, 25th, 75th, and 95th percentiles were calculated. Results: Nuchal translucency measurements varied considerably with gestational age; this variation followed a fetus-specific pattern. In 94% of cases, we observed an increase followed by a steady decrease in nuchal translucency measurement. A visible nuchal translucency was found after 76 and 86 days' gestation in 97% (95% confidence interval [CI] 89, 100) and 100% (95% CI 94, 100) of the fetuses, respectively. The median nuchal translucency increased from 0.7 mm at 70 days' gestation to 1.7 mm at 91 days' gestation, after which it declined to 1.0 mm at 105 days' gestation. Conclusion: A progressive increase and subsequent decrease in nuchal translucency thickness occurs with advancing gestation in most fetuses, but the timing of the peak thickening appears to be fetus-specific. In this study, each fetus developed a visible nuchal translucency. If the nuchal translucency measurement is 0 mm before 12 weeks, it may be advisable to repeat the measurement at 12 weeks' gestation. In contrast, a nuchal translucency that cannot be measured from 12 weeks' gestation onward suggests that this temporary anatomic entity is already in its waning phase.
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Nuchal translucency
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Nuchal translucency
Nuchal Translucency Measurement
Biparietal diameter
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Nuchal translucency
Nuchal Translucency Measurement
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Screening of pregnancies for trisomy 21 is now an accepted part of antenatal care. Measurement of fetal nuchal translucency in the first trimester and analysis of maternal serum biochemistry in the second trimester are both established methods of screening. The performance characteristics of both tests in an unselected population are well described and the choice of test offered is usually determined by local policy and resources. We present data from a screening programme offering women with a low risk result from nuchal translucency measurement a second trimester serum screen. There were eight cases of trisomy 21 in the 2683 women screened, all of which presented with a high-risk nuchal screen result. Serum screening of 1057 women who screened negative by nuchal translucency gave 46 high-risk results, all of which were, therefore, false positive for trisomy 21. Second trimester biochemistry screening following a negative nuchal translucency screen did not increase the detection of trisomy 21.
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Trisomy
Nuchal Translucency Measurement
Second trimester
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With the advent of nuchal translucency as a screening test for trisomy 21, increasing numbers of women are having an ultrasound scan between 11 and 14 weeks. The RCOG Working Party (July 2000) recommendations for ultrasound in pregnancy do not mention a first trimester scan for nuchal translucency or fetal anatomy. However, this scan is not just an opportunity to perform nuchal translucency; many structural abnormalities can also be seen at this stage.
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Nuchal translucency thickness (NT) measurement as a screening test for fetal Down syndrome has revolutionized obstetric ultrasound. For the first time, the performance and usefulness of a sonographic marker (NT) as a screening test for a fetal condition (trisomy 21) was tested prospectively and confirmed by many large-scale studies using the same or very similar methodology, prior to being accepted generally and incorporated into routine clinical practice. Based on these studies, it became very clear that the success of using a sonographic marker as a screening tool requires the development of a precise protocol, proper operator training, certification, auditing of performance and re-certification. When any of these elements is lacking, the high accuracy of NT screening is not achievable. The importance of adherence to a strict protocol and stringent quality assurance has been recognised for decades by our colleagues in the biochemistry department through their development of maternal serum screening for open neural tube defects and Down syndrome. In contrast, many sonographic tests, which are actually far more complicated and cumbersome than NT measurement, have been incorporated widely into clinical practice well before full evaluation, leading to much confusion and mismanagement. Of these, the second-trimester ‘soft markers’ is the best example. Soft markers are qualitatively different from structural anomalies. They are detected in a variable proportion of normal fetuses. They are of interest because of their statistical association with chromosomal abnormalities, making such markers potentially useful as a screening tool for fetal aneuploidies. Since the majority of cases of trisomies 13 and 18 are associated with structural anomalies that are detectable by ultrasound, the role of soft markers for screening of these conditions is limited. Therefore, the following discussion focuses on the clinical value of using soft markers as a screening test for trisomy 21. Unlike NT, the typical history of soft markers started with the documentation of a new proposed marker in a series of abnormal cases in a high-risk population, without controlling for confounding factors. The subsequent overwhelming enthusiasm led to the premature incorporation of assessment of the marker into routine clinical practice. Only with more experience did the denominator emerge which lowers the incidence and strength of association. New technologies and studies in ultrasound then showed the marker to be present in far more normal cases than was originally thought, reducing the positive predictive value further, even as far as to the background risk. Eventually, only after being used for several years, have many of these markers been found to be poor or even useless for trisomy 21 screening. Although ultrasound is a ‘non-invasive’ procedure, it is not without risk. Pregnant women referred for further assessment because of a soft marker have significantly higher levels of anxiety than do those referred for advanced maternal age only, at a level similar to those referred for abnormal maternal serum biochemistry1. This anxiety could lead to a cascade of additional, often unnecessary, tests and is often very difficult to resolve without an invasive diagnostic test. Therefore, all sonographic tests, like any medical test, must be evaluated properly, and should only be implemented if a clear benefit is demonstrated. So, how should we assess the usefulness of soft markers? If they are being used for the purpose of screening for trisomy 21, they should be examined as such. In other words, we should be satisfied that we have a well-characterized test that provides an acceptable sensitivity and specificity. Unfortunately, none of the soft markers used in clinical practice has undergone adequate vigorous scientific scrutiny, and the major problems are as follows. Firstly, the second-trimester ‘genetic sonogram’, which includes both soft markers and structural anomalies, when used by the most experienced operators was able to detect about 70% of cases of trisomy 21, with a false-positive rate (FPR) of about 10%2. This performance would be substantially lower if only soft markers were included, and is substantially inferior to other well-established screening protocols, such as first-trimester NT screening (detection rate of 76.8% at a 4.2% FPR3) or first-trimester combined screening (detection rate approaching 90%4). Therefore, it is quite clear that second-trimester soft markers alone should not be the screening test of choice for trisomy 21. Secondly, do we have a standardized methodology in applying soft markers as a screening test for trisomy 21? The answer, quite simply, is ‘No’. There are wide variations in the definition of individual markers; the methodology of examination is not standardized; and many of the reported studies were performed in referral centers on high-risk pregnancies. It is generally agreed that assessment of multiple markers is required to produce acceptable screening performance. However, there is no consensus as to how many markers should be examined, or how to integrate these markers. Some have proposed the simple approach of defining a high-risk group by the presence of two or more markers, or using a scoring system5, but there is much controversy over the relative importance of each marker and therefore the validity of such a simple approach. In contrast, the use of likelihood ratios (LR) appears to be a more scientific approach. The use of LR enables us to modify the background risk, taking into account all of the available information including prior screening test results. Unfortunately, there are wide variations in the reported LRs associated with each of the markers individually or in combination, due to the lack of consistent definitions and methodologies in their assessment. For example, the estimated LR for isolated echogenic intracardiac focus ranges from 1.1 (useless) to 5.4 (moderately strong marker)6-8. Which is correct? If we are going to use several markers, the ultimate uncertainty in the combined LR could be enormous. Thirdly, the absence of soft markers has been used to reduce the individualized risk of trisomy 21, with a reported negative LR as low as 0.159. The reported series are in general conducted in very specialized centers. It is still unclear what the exact negative LR should be, how many markers need to be included in the assessment, and the precise definition of each marker. What is certain, however, is that reproducible results cannot be achieved until the same methodology is used consistently. Therefore, it is recommended that a reduction of risk should not be applied ‘on the basis of a 16–20-week ‘screening’ scan, owing to the variety of imaging locations involved’10, unless the scan is ‘undertaken in an established centre performing tertiary-level ultrasound’11. We would add that such centers should either have the data to generate their own negative LR, or, if they are using reported negative LRs from the medical literature, should have adequate evidence to confirm that the methodology used is exactly the same as that described in the original reports, and that their performances are comparable. Fourthly, unlike first-trimester NT screening, there is no specific structured training for the assessment of each of these soft markers, nor is there any certification or recertification process. It is unlikely that adequate quality assurance programs are present in the majority of centers to audit the outcome and performance of the center or of individuals in using these soft markers. Experience from all screening programs, including first-trimester NT screening as the closest example, tells us that no screening program will be successful in the absence of a stringent certification and quality assurance process. It is quite clear that if a woman requests prenatal screening for trisomy 21, sonographic soft markers would not be the test of choice. Yet, there is a great temptation to use second-trimester sonographic markers to modify the individualized risk of trisomy 21 in patients who have received other forms of screening tests in early pregnancy. However, it does not seem logical to modify a risk calculated on the basis of a reliable test, which was well studied and had a clear protocol and quality assurance program, by a second test that lacks standardization or quality assurance—possibly with the exception of the few units which have considerable follow-up data of their own12, 13. We have no intention of disputing the association between second-trimester markers and fetal trisomy 21, in particular nasal bone hypoplasia, echogenic bowel and nuchal fold thickness. However, we do have concerns that these markers are being used in routine clinical settings before their effect on screening has been fully elucidated, with an uncertain effect on the overall screening performance. With the many effective screening programs currently available in the first and early second trimesters, it is likely that only very strong soft markers, such as those with a LR of more than 10, will contribute significantly to further improving the performance of trisomy 21 screening programs. Therefore, efforts should be focused on: (i) the development of a well-defined protocol for the evaluation of these few strong markers, including how to do it, when to do it, and who should do it; (ii) the development of an algorithm for incorporating these markers into existing screening programs, such as in the recent study by Borrell et al.14; and (iii) the confirmation of their efficacy as screening markers by large prospective studies in an unselected population. Over the last few years, we have been pleased to see that there have been more discussions in the medical literature concerning training and quality assessment in obstetric sonography15-17. This is encouraging. It should be remembered that a service without quality is worse than no service at all. We would like to conclude with the following quote: ‘The screening procedure should not be merely a test but must be a comprehensive program… . The screening process must allow patients to decline intervention at each step throughout the process. A screening program must undertake regular clinical audit to evaluate local performance’11. There is still much to be achieved before second-trimester soft markers find a definite role in fetal aneuploidy screening. The basic principle of medicine—‘First do no harm’—should always be observed. Any test, be it diagnostic or screening in nature, should not be used in clinical practice unless its efficacy has been established.
Nuchal translucency
Nuchal Translucency Measurement
Second trimester
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In Brief Screening for Down syndrome is an important part of routine antenatal care. The most common screening method in the United States involves the assessment of a combination of factors: maternal age, multiple second-trimester serum markers, and second-trimester ultrasonography (as a so-called “genetic sonogram”). More recently, however, there has been significant interest in first-trimester methods of screening, including screening for first-trimester serum markers and the sonographic measurement of fetal nuchal translucency. Multiple studies have demonstrated that fetal nuchal translucency has the potential of being a very powerful predictor of fetal aneuploidy. However, for clinicians a large void remains between this knowledge and the practical issues that must be addressed prior to endorsing this form of screening for widespread use. This article provides an objective assessment of the literature describing nuchal translucency, as well as some adjunct first-trimester sonographic techniques, such as ductus venosus flow and nasal bone studies. Additionally, a detailed description of practical problems that might limit the implementation of this form of screening is presented. First-trimester nuchal translucency sonography represents a great advance in the field of Down syndrome screening, although many practical implementation issues remain to be clarified prior to recommending widespread utilization of this technique.
Ductus venosus
Nuchal Translucency Measurement
Nuchal translucency
Nasal bone
Second trimester
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To assess the learning curves for nuchal translucency thickness as measured by both manual and semi-automated methods. Fetal nuchal translucency was obtained by an experienced sonologist according to the FMF guidelines in a cohort of 32 consecutive first trimester fetuses at 11–13 weeks, and calculated by both manual and semi-automated methods (GE Voluson E6). Subsequently, two sonographer trainees with no prior experience in first trimester ultrasound examined archived images in separated occasions and measured the nuchal translucency first by the manual method and secondly with the semi-automated method. Each of the trainees was blinded to any pre-existing results. The average difference between the two trainees and the expert in both manual and semi-automated methods was calculated. A difference below 15% was considered as an accurate measurement. The average learning curve was delineated using the cumulative sum analysis (CUSUM). CUSUM plots demonstrated that the mean learning curve was achieved by 19 and 25 measurements for semi-automated and manual methods, respectively. For an inexperienced sonographer to become competent in nuchal translucency measurement in stored images, a minimum number of 19 and 25 measurements was required for the semi-automated and manual method, respectively.
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CUSUM
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