The 11-14 weeks scan - KH Nicolaides, NJ Sebire, RJM Snijders, AP Souka

Chapter 1

NUCHAL TRANSLUCENCY THICKNESS

Cystic hygromas, nuchal edema and nuchal translucency

During the second and third trimesters of pregnancy, abnormal accumulation of fluid behind the fetal neck can be classified as nuchal cystic hygroma or nuchal edema. In about 75% of fetuses with cystic hygromas, there is a chromosomal abnormality and, in about 95% of cases, the abnormality is Turner syndrome. Nuchal edema has a diverse etiology; chromosomal abnormalities are found in about one-third of the fetuses and, in about 75% of cases, the abnormality is trisomy 21 or 18. Edema is also associated with fetal cardiovascular and pulmonary defects, skeletal dysplasias, congenital infection and metabolic and hematological disorders; consequently, the prognosis for chromosomally normal fetuses with nuchal edema is poor.

In the first trimester, the term translucency is used, irrespective of whether it is septated or not and whether it is confined to the neck or envelopes the whole fetus.69 During the second trimester, the translucency usually resolves and, in a few cases, it evolves into either nuchal edema or cystic hygromas with or without generalized hydrops. Neither the incidence of chromosomal defects nor the prognosis can be predicted by the ultrasonographic appearance of the lesion. Increased NT is associated with trisomy 21, Turner syndrome and other chromosomal abnormalities as well as many fetal malformations and genetic syndromes. The incidence of these abnormalities is related to the thickness, rather than the appearance, of NT. Furthermore, it is possible to standardize and audit the results of a measurement but not those of a subjective appearance.

Nuchal translucency - definition

• Nuchal translucency is the sonographic appearance of subcutaneous accumulation of fluid behind the fetal neck in the first trimester of pregnancy
• The term translucency is used, irrespective of whether it is septated or not and whether it is confined to the neck or envelopes the whole fetus.
• The incidence of chromosomal and other abnormalities is related to the size, rather than the appearance of nuchal translucency.
• During the second trimester, the translucency usually resolves and, in a few cases, it evolves into either nuchal edema or cystic hygromas with or without generalized hydrops.

Measurement of nuchal translucency

The ability to achieve a reliable measurement of NT is dependent on appropriate training and adherence to a standard technique in order to achieve uniformity of results among different operators.

Gestation and crown-rump length

The optimal gestational age for measurement of fetal NT is 11 weeks to 13 weeks and 6 days. The minimum fetal crown–rump length should be 45 mm and the maximum 84 mm.

The reasons for selecting 13 weeks and 6 days as the upper limit are firstly, to provide women with affected fetuses the option of first rather than second trimester termination, secondly, the incidence of abnormal accumulation of nuchal fluid in chromosomally abnormal fetuses is lower at 14-18 weeks than before 14 weeks, and thirdly, the success rate for taking a measurement at 10-13 weeks is 98–100%, falling to 90% at 14 weeks because the fetus becomes vertical making it more difficult to obtain the appropriate image.

The reason for selecting 10 weeks as the earliest gestation was because screening necessitates the availability of a diagnostic test and in the early 1990’s it was appreciated that chorionic villous sampling before 10 weeks was associated with transverse limb reduction defects. It was subsequently realized that many major fetal abnormalities can be diagnosed at the NT scan, provided the minimum gestation is 11 weeks. For example, diagnosis or exclusion of acrania and therefore anencephaly, cannot be made before 11 weeks because sonographic assessment of ossification of the fetal skull is not reliable before this gestation. Examination of the four-chamber view of the heart and main arteries is possible only after 10 weeks. At 8-10 weeks all fetuses demonstrate herniation of the midgut that is visualized as a hyperechogenic mass in the base of the umbilical cord, and it is therefore unsafe to diagnose or exclude exomphalos at this gestation. The fetal bladder can be visualized in only 50% of fetuses at 10 weeks, in 80% at 11 weeks and in all cases by 12 weeks.

Image and measurement

In the assessment of fetal NT the ultrasound machine should be of high resolution with a video-loop function and callipers that provide measurements to one decimal point. Fetal NT can be measured successfully by transabdominal ultrasound examination in about 95% of cases; in the others, it is necessary to perform transvaginal sonography. The results from transabdominal and transvaginal scanning are similar.

Only the fetal head and upper thorax should be included in the image for measurement of NT (Figure 8a). The magnification should be as large as possible and always such that each slight movement of the callipers produces only a 0.1 mm change in the measurement. In magnifying the image, either pre or post freeze zoom, it is important to turn the gain down. This avoids the mistake of placing the calliper on the fuzzy edge of the line which causes an underestimate of the nuchal measurement.

A good sagittal section of the fetus, as for measurement of fetal crown–rump length, should be obtained and the NT should be measured with the fetus in the neutral position. When the fetal neck is hyperextended the measurement can be increased by 0.6 mm and when the neck is flexed, the measurement can be decreased by 0.4 mm.

Care must be taken to distinguish between fetal skin and amnion because, at this gestation, both structures appear as thin membranes (Figure 8a). This is achieved by waiting for spontaneous fetal movement away from the amniotic membrane; alternatively, the fetus is bounced off the amnion by asking the mother to cough and/or by tapping the maternal abdomen.

The maximum thickness of the subcutaneous translucency between the skin and the soft tissue overlying the cervical spine should be measured (Figure 8e). The callipers should be placed on the lines that define the NT thickness - the crossbar of the calliper should be such that it is hardly visible as it merges with the white line of the border and not in the nuchal fluid. During the scan, more than one measurement must be taken and the maximum one should be recorded.

The umbilical cord may be round the fetal neck in 5–10% of cases and this finding may produce a falsely increased NT. In such cases, the measurements of NT above and below the cord are different and, in the calculation of risk, it is more appropriate to use the average of the two measurements (Figure 8f).

There are no clinically relevant effects on NT measurements by ethnic origin, parity or gravidity, cigarette smoking, diabetic control, conception by assisted reproduction techniques, bleeding in early pregnancy or fetal gender.

The intra-observer and inter-observer differences in measurements of fetal NT are less than 0.5 mm in 95% of cases.

Deviation in measurement from normal

Fetal NT increases with crown–rump length and therefore it is essential to take gestation into account when determining whether a given NT thickness is increased. In a study involving 96,127 pregnancies, the median and 95th centile at a crown-rump-length of 45 mm were 1.2, and 2.1 mm and the respective values at crown rump length of 84 mm were 1.9 and 2.7 mm (Snijders et al 1998).

In screening for chromosomal defects patient-specific risks are derived by multiplying the a priori maternal age and gestation-related risk by a likelihood ratio, which depends on the difference (Delta value in mm) in fetal NT measurement from the expected normal median for the same crown-rump length (Figures 9-11).

In screening using maternal serum biochemical markers a different approach has been used to take into account the gestational age related change in marker levels. This method involves converting the measured concentration into a multiple of the median (MOM) of unaffected pregnancies of the same gestation. Essentially, the Gaussian distributions of log10 (NT MoM) in trisomy 21 and unaffected pregnancies are derived and the heights of the distributions at a particular MoM, which is the likelihood ratio for trisomy 21, is used to modify the a priori maternal age-related risk to derive the patient-specific risk.

In screening by NT the Delta approach provides accurate patient-specific risks (Spencer et al 2003a). In contrast, the MoM approach was found to be inappropriate for this purpose, because none of the three basic assumptions that underpin this method are valid. Firstly, in the unaffected population the distributions of NT MoM and log10(NT MoM) were not Gaussian, secondly, the SD’s did not remain constant with gestation and thirdly, the median MoM in the trisomy 21 pregnancies was not a constant proportion of the median for unaffected pregnancies. The MoM approach resulted in women being given an overestimate of risk for trisomy at 11 weeks and a considerable underestimate of risk at 13 weeks.

Nuchal translucency - measurement

• The gestation should be 11-13+6 weeks and the fetal crown–rump length should be 45-84 mm.
• A mid-sagittal section of the fetus should be obtained and the NT should be measured with the fetus in the neutral position.
• Only the fetal head and upper thorax should be included in the image. The magnification should be as large as possible and always such that each slight movement of the callipers produces only a 0.1 mm change in the measurement.
• The maximum thickness of the subcutaneous translucency between the skin and the soft tissue overlying the cervical spine should be measured. Care must be taken to distinguish between fetal skin.
• The callipers should be placed on the lines that define the NT thickness - the crossbar of the calliper should be such that it is hardly visible as it merges with the white line of the border and not in the nuchal fluid.
• During the scan, more than one measurement must be taken and the maximum one should be recorded.

Training and quality assessment in the measurement of nuchal translucency

Appropriate training of sonographers and adherence to a standard technique for the measurement of NT are essential prerequisites for good clinical practice. Furthermore, the success of a screening program necessitates the presence of a system for regular audit of results and continuous assessment of the quality of images. In 1997, a Study Group of the Royal College of Obstetricians and Gynaecologists in the UK recommended that NT screening should only be conducted by highly competent sonographers, certified by an external agency and subject to external quality assurance and ongoing audit.

All sonographers performing fetal scans should be capable of reliably measuring the crown–rump length and obtaining a proper sagittal view of the fetal spine. For such sonographers, it is easy to acquire, within a few hours, the skill to measure NT thickness. However, the ability to measure NT and obtain reproducible results improves with training. Good results are achieved after 80 scans for the transabdominal route and 100 scans transvaginally.

The Fetal Medicine Foundation (FMF), which is a UK registered charity, has established a process of training and quality assurance for the appropriate introduction of NT screening into clinical practice. Training is based on a theoretical course, practical instruction on how to obtain the appropriate image and make the correct measurement of NT, and presentation of a logbook of images. These are examined to determine if the magnification is adequate, the section of the fetus is truly sagittal and the head is in the neutral position, the amnion is seen separately from the nuchal membrane and the calipers are placed appropriately.

Ongoing quality assurance is based on assessment of the distribution of fetal NT measurements (Figure 12) and examination of a sample of images obtained by each sonographer involved in screening.

Several studies have demonstrated that ongoing regular audit of images and the distribution of measurements of NT is essential for assessing the quality of a centre and is also useful in identifying individual sonographers whose results deviate from the mean performance. The inter-examination variation in measurements is reduced considerably after an initial learning phase and after feedback to the sonographers on the distribution of their measurements and the quality of their images.

Nuchal translucency: Training and quality assurance

• Appropriate training of sonographers and adherence to a standard technique for the measurement of NT are essential prerequisites for good clinical practice.
• The success of a screening program necessitates the presence of a system for regular audit of results and continuous assessment of the quality of images.
• Training is based on a theoretical course, practical instruction on how to obtain the appropriate image and make the correct measurement of NT, and presentation of a logbook of images.
• Ongoing quality assurance is based on assessment of the distribution of fetal NT measurements and examination of a sample of images obtained by each sonographer involved in screening.

Nuchal translucency thickness and risk for chromosomal defects

In 1992 a study, in which fetal NT was measured before CVS for fetal karyotyping, reported that in a high proportion of chromosomally abnormal fetuses the NT thickness was increased (Nicolaides et al 1992). This association was subsequently confirmed in several other studies in the early 1990s. Thus, in the combined data from 17 series involving a total of 1,690 patients with increased fetal NT the incidence of chromosomal defects was 28.7% (Nicolaides 2004). However, there were large differences between the studies in the incidence of chromosomal defects, ranging from 11% to 88%, because of differences in the maternal age distributions of the populations examined and the definition of the minimum abnormal NT thickness, which ranged from 2 mm to 10 mm.

Studies in the mid 1990’s demonstrated that firstly, in normal pregnancies, fetal NT thickness increases with gestation, secondly, in trisomy 21 and other major chromosomal defects fetal NT is increased, and thirdly, the risk for trisomies can be derived by multiplying the a priori maternal age and gestation-related risk by a likelihood ratio, which depends on the degree of deviation in fetal NT measurement from the expected normal median for that crown–rump length (Nicolaides et al 1994, Pandya et al 1995. It was estimated that, in a pregnant population with a mean maternal age of 28 years, using the risk cut-off of 1 in 300 to define the screen positive group would detect about 80% of trisomy 21 fetuses for a false positive rate of 5%.).

Nuchal translucency: Calculation of patient-specific risk

• The risk for trisomies is derived by multiplying the a priori maternal age and gestation-related risk by the NT likelihood ratio
• The NT likelihood ratio depends on the degree of deviation in fetal NT measurement from the expected normal median for that crown–rump length

Implementation of nuchal translucency screening in routine practice

Several prospective interventional studies have examined the implementation of NT screening in routine practice (Nicolaides 2004). In some of the studies the screen positive group was defined by a cut-off in fetal NT or a combined risk derived from the maternal age and deviation in fetal NT from the normal median for fetal crown-rump length.

The important findings of these studies are, firstly, fetal NT was successfully measured in more than 99% of cases, secondly, there were inevitable variations in false positive and detection rates between the studies because of differences in the maternal age distribution of their populations and in fetal NT or risk cut-offs used, and thirdly, in the combined data on more than 200,000 pregnancies, including more than 900 fetuses with trisomy 21, fetal NT screening identified more than 75% of fetuses with trisomy 21 and other major chromosomal defects for a false positive rate of 5%, or the detection rate was about 60% for a false positive rate of 1% (Nicolaides 2004).

In the largest study, coordinated by the FMF, 100,311 singleton pregnancies were examined by 306 appropriately trained sonographers in 22 UK centers (Snijders et al 1998). In all cases the fetal NT and crown–rump length were measured and individual patient-specific risks, based on maternal age, gestational age and fetal NT were calculated. Follow-up was obtained from 96,127 cases, including 326 with trisomy 21 and 325 with other chromosomal abnormalities (Table 3). The median gestation at the time of screening was 12 weeks (range 10–14 weeks) and the median maternal age was 31 years. The estimated risk for trisomy 21 was above 1 in 300 in 8% of the normal pregnancies, in 82% of those with trisomy 21 and in 78% with other chromosomal abnormalities. For a screen-positive rate of 5%, the detection rate was 77% (95% confidence interval 72–82%).

Table 3. Multicentre study coordinated by the Fetal Medicine Foundation. Number of pregnancies with nuchal translucency (NT) thickness above the 95th centile and an estimated risk for trisomy 21, based on maternal age and fetal nuchal translucency and crown-rump length, of 1 in 300 or more (Snijders et al 1998).

* Deletions, partial trisomies, unbalanced translocations, sex chromosome aneuploidies

The issue of fetal lethality

Screening for chromosomal defects in the first, rather than the second trimester, has the advantage of earlier prenatal diagnosis and consequently less traumatic termination of pregnancy for those couples who choose this option. A potential disadvantage is that earlier screening preferentially identifies those chromosomally abnormal pregnancies that are destined to miscarry. Approximately 30% of affected fetuses die between 12 weeks of gestation and term. This issue of preferential intrauterine lethality of chromosomal defects is, of course, a potential criticism of all methods of antenatal screening, including second-trimester maternal serum biochemistry, because the rate of intrauterine lethality between 16 weeks and term is about 20%.

In prenatal screening studies it is impossible to know how many of the trisomy 21 pregnancies that were terminated would have resulted in live births. However, it is possible to estimate the impact of prenatal screening on the prevalence of trisomy 21 in live births. This can be done by comparing the number of affected live births with the number estimated on the basis of the maternal age-related prevalence of trisomy 21 in live births and the maternal age distribution of the population screened. In the FMF screening study, by a combination of maternal age and fetal NT, a risk cut-off of 1 in 300 was associated with a false positive rate of 8.3% and a detection rate of 82.2% (Snijders et al 1998). It was estimated that prenatal screening followed by invasive diagnostic testing and selective termination of affected fetuses would have reduced the potential live birth prevalence of trisomy 21 by 78-82%.

Nuchal translucency: Effectiveness of screening for trisomy 21

• Prospective studies in more than 200,000 pregnancies, including more than 900 fetuses with trisomy 21, have demonstrated that NT screening can identify more than 75% of fetuses with trisomy 21 for a false positive rate of 5%.
• Increased NT does not necessarily identify those trisomic fetuses that are destined to die in utero.
• The observed detection rate of trisomy 21 with first trimester NT screening is only 2-3% higher than the detection rate of affected pregnancies that would potentially result in livebirths.

Observational studies

The ability to achieve a reliable measurement of NT is dependent on appropriate training, adherence to a standard technique and motivation of the sonographer. All three components are well illustrated by the differences in results between interventional and observational studies, in which the sonographers were asked to record the fetal NT measurements but not act on the results (Nicolaides 2004). Thus, successful measurement of NT was achieved in more than 99% of cases in the interventional studies, but in only 75% of cases in the observational studies. Furthermore in the interventional studies there was increased NT in 76.8% of the trisomy 21 and 4.2% of the chromosomally normal fetuses, compared to the respective rates of 38.4% and 5.0% in the observational studies.

In the observational studies, the scans were often carried out at inappropriate gestations and the sonographers were either not trained adequately or they were not sufficiently motivated to measure NT. For example, in one of the studies, in which sonographers were instructed to take no extra scanning time other than that necessary for measurement of the crown-rump length, fetal NT was successfully measured in only 66% of cases (Roberts et al 1995). In another study, the fetal crown-rump length was less than 33 mm in 54% of cases and the sonographers, who were instructed to measure fetal NT within three minutes, were unable to do so in 42% of cases (Kornman et al 1996). These methodological problems are further highlighted by a study of 47,053 singleton pregnancies examined at 6-16 weeks (Wald et al 2003a). In 23% of the patients no valid NT measurement was taken because the scans were carried out at inappropriate gestations or the sonographers were unable to obtain a measurement or none of the images were deemed to be of an acceptable quality.

Further evidence on the difference between observational and interventional studies is provided by Crossley et al (2002). In this observational study, 17,229 pregnancies were recruited and fetal NT was successfully measured in 73% of cases. In a subsequent study of more than 2,000 pregnancies in which the results of the scan were given to the women, fetal NT was successfully measured in 99.8% of cases.

Nuchal translucency and maternal serum biochemistry

Trisomic pregnancies are associated with altered maternal serum concentrations of various feto-placental products, including AFP, free β-hCG, unconjugated estriol (uE3), Inhibin A and pregnancy associated plasma protein A (PAPP-A).

Screening in the second trimester by maternal age and various combinations of free β-hCG, AFP, uE3 and Inhibin A can identify 50-75% of trisomy 21 pregnancies for a false positive rate of 5%. Screening in the first trimester by a combination of maternal age and serum free β-hCG and PAPP-A identifies about 60% of affected pregnancies for a false positive rate of 5%. However, an essential component of biochemical screening is accurate dating of the pregnancy by ultrasound, otherwise the detection rate is reduced by about 10%.

Fetal NT and maternal serum testing in the first-trimester

In trisomy 21 pregnancies at 12 weeks, the maternal serum concentration of free β-hCG (about 2 MoM) is higher than in chromosomally normal fetuses whereas PAPP-A is lower (about 0.5 MoM). The difference in maternal serum free β-hCG between normal and trisomy 21 pregnancies increases with advancing gestation and the difference in PAPP-A decreases with gestation. These temporal variations in marker levels, their interrelation and their association with maternal weight should be taken into account when developing risk algorithms in order to produce accurate patient-specific risks.

There is no significant association between fetal NT and maternal serum free β-hCG or PAPP-A in either trisomy 21 or chromosomally normal pregnancies and therefore the ultrasononographic and biochemical markers can be combined to provide more effective screening than either method individually (Spencer et al 1999).

Six prospective screening studies have confirmed the feasibility and effectiveness of combining fetal NT and maternal serum free β-hCG and PAPP-A. In the combined data on a total of 38,804 pregnancies, including 182 with trisomy 21, the detection rate for trisomy 21 at a 5% false positive rate was 86.3% (Nicolaides 2004).

In trisomies 18 and 13 maternal serum free β-hCG and PAPP-A are decreased. In cases of sex chromosomal anomalies maternal serum free β-hCG is normal and PAPP-A is low. In diandric triploidy maternal serum free β-hCG is greatly increased, whereas PAPP-A is mildly decreased. Digynic triploidy is associated with markedly decreased maternal serum free β-hCG and PAPP-A. Screening by a combination of fetal NT and maternal serum PAPP-A and free β-hCG can identify about 90% of all these chromosomal abnormalities for a screen positive rate of 1%.

An important development in biochemical analysis is the introduction of a new technique (random access immunoassay analyzer using time-resolved-amplified-cryptate-emission), which provides automated, precise and reproducible measurements within 30 minutes of obtaining a blood sample. This has made it possible to combine biochemical and ultrasonographic testing as well as to counsel in one-stop clinics for early assessment of fetal risk (OSCAR) (Bindra et al 2002, Spencer et al 2003b).

Fetal NT and maternal serum testing in the second-trimester

In women having second-trimester biochemical testing following first-trimester NT screening the a priori risk needs to be adjusted to take into account the first-trimester screening results. Prospective studies of screening by a combination of fetal NT in the first trimester and maternal serum biochemistry in the second trimester reported that for a false positive rate of 5% the detection rate of trisomy 21 (85-90%) is similar to combined screening in the first trimester (Nicolaides 2004).

Integration of first and second trimester testing

A statistical model combining first-trimester fetal NT and maternal serum PAPP-A with second-trimester free β-hCG, estriol and inhibin A, estimated that for a false positive rate of 5% the detection rate of trisomy 21 could be 94% (Wald et al 1999). This test assumes complete compliance by the pregnant women in firstly, participating in a two stage process separated by one month, secondly, in having an ultrasound scan without receiving information as to whether the fetus looks normal or not, and thirdly, accepting second rather than first trimester diagnosis and termination. It is therefore likely that even if the estimates of this hypothetical test are found to be true in prospective studies, it will not gain widespread clinical acceptability.

Some of the logistical problems in the implementation of an integrated test are highlighted by the results of a multicentre observational study (SURUSS) investigating first and second trimester screening for trisomy 21 (Wald et al 2003a). The aim was to obtain a measurement of fetal NT in the first trimester and collect maternal serum and urine samples in the first and second trimesters. Intervention was based on the second trimester serum results and all other data were analyzed retrospectively. However, of the 47,053 women that were recruited, only 60% completed all components of the protocol. In this study there were 101 fetuses with trisomy 21 and satisfactory NT images were obtained only from 75 of the cases. The data were used to derive a statistical model suggesting that for a 5% false positive rate, 93% of trisomy 21 fetuses could be detected by the integrated test. However, it is likely that this model is inaccurate. For example, the predicted detection rates, for a 5% false positive rate, were 71% for the double test, 77% for the triple test and 83% for the quadruple test, which are substantially higher than the respective rates of 61%, 66% and 75% reported by the same authors in their prospective screening studies (Wald et al 2003b).

A similar study in the USA (FASTER trial), reported its findings in the subgroup of 33,557 pregnancies with complete first and second trimester data, including 84 cases of trisomy 21 (Malone et al 2004). It was estimated that, for a 5.4% false positive rate, 90% of trisomy 21 fetuses could be detected.

Prospective studies have demonstrated that such results are achievable by screening with fetal NT and maternal serum free β-hCG and PAPP-A in the first-trimester (Bindra et al 2002, Spencer et al 2003b).

Screenining by nuchal translucency and serum biochemistry

• In trisomy 21 pregnancies at 11-13+6 weeks, the maternal serum concentration of free β-hCG is higher (about 2 MoM) and PAPP-A is lower (about 0.5 MoM) than in chromosomally normal fetuses
• There is no significant association between fetal NT and maternal serum free β-hCG or PAPP-A in either trisomy 21 or chromosomally normal pregnancies. The ultrasononographic and biochemical markers can be combined to provide more effective screening than either method individually
• Prospective studies, in more than 50,000 pregnancies, including more than 250 fetuses with trisomy 21, have demonstrated that screening by a combination of fetal NT and either first or second trimester maternal serum biochemistry can identify 85-90% of fetuses with trisomy 21 for a false positive rate of 5%.
• In trisomies 18 and 13 maternal serum free β-hCG and PAPP-A are decreased. In sex chromosomal anomalies maternal serum free β-hCG is normal and PAPP-A is low. In diandric triploidy maternal serum free β-hCG is greatly increased, whereas PAPP-A is mildly decreased. Digynic triploidy is associated with markedly decreased maternal serum free β-hCG and PAPP-A. Screening by a combination of fetal NT and maternal serum PAPP-A and free β-hCG can identify about 90% of all these chromosomal abnormalities for a screen positive rate of 1%.