Screening for fetal aneuploidies at 11 to 13 weeks
Effective screening for major aneuploidies can be provided in the first trimester of pregnancy. Screening by a combination of fetal nuchal translucency and maternal serum free-β-human chorionic gonadotrophin and pregnancy-associated plasma protein-A can identify about 90% of fetuses with trisomy 21 and other major aneuploidies for a false-positive rate of 5%. Improvement in the performance of first-trimester screening can be achieved by firstly, inclusion in the ultrasound examination assessment of the nasal bone and flow in the ductus venosus, hepatic artery and across the tricuspid valve, and secondly, carrying out the biochemical test at 9 to 10 weeks and the ultrasound scan at 12 weeks. Copyright © 2011 John Wiley & Sons, Ltd.
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Aneuploidies are major causes of perinatal death and childhood handicap. Consequently, the detection of chromosomal disorders constitutes the most frequent indication for invasive prenatal diagnosis. However, invasive testing, by amniocentesis or chorionic villus sampling (CVS), is associated with a risk of miscarriage, and therefore these tests are carried out only in pregnancies considered to be at high risk for aneuploidies.
In the 1970s, the main method of screening for aneuploidies was by maternal age and in the 1980s by maternal serum biochemistry and detailed ultrasonographic examination in the second trimester. In the 1990s, the emphasis shifted to the first trimester when it was realized that the great majority of fetuses with major aneuploidies can be identified by a combination of maternal age, fetal nuchal translucency (NT) thickness and maternal serum free β-human chorionic gonadotrophin (β-hCG) and pregnancy-associated plasma protein-A (PAPP-A). In the last 10 years, several additional first-trimester sonographic markers have been described which improve the detection rate of aneuploidies and reduce the false-positive rate. The performance of the different methods of screening for trisomy 21 is summarized in Table 1.
Table 1. Performance of different methods of screening for trisomy 21
|First trimester|| || |
|MA + fetal NT||75–80||5|
|MA + serum free β-hCG and PAPP-A||60–70||5|
|MA + NT + free β-hCG and PAPP-A (combined test)||85–95||5|
|Combined test + nasal bone or tricuspid flow or ductus venosus flow||93–96||2.5|
|Second trimester|| || |
|MA + serum AFP, hCG (double test)||55–60||5|
|MA + serum AFP, free β-hCG (double test)||60–65||5|
|MA + serum AFP, hCG, uE3 (triple test)||60–65||5|
|MA + serum AFP, free β-hCG, uE3 (triple test)||65–70||5|
|MA + serum AFP, hCG, uE3, inhibin A (quadruple test)||65–70||5|
|MA + serum AFP, free β-hCG, uE3, inhibin A (quadruple test)||70–75||5|
|MA + NT + PAPP-A (11–13 weeks) + quadruple test||90–94||5|
SCREENING BY MATERNAL AGE
The risk for many aneuploidies increases with maternal age. Additionally, because aneuploid fetuses are more likely to die in utero than euploid fetuses, the risk decreases with gestation.
The rate of fetal death between 12 weeks (when first-trimester screening is performed) and term is about 30% for trisomy 21 and 80% for trisomies 18 and 13. (Hecht and Hook, 1994; Halliday et al., 1995; Snijders et al., 1994, 1995, 1999; Morris et al., 1999). In contrast, the rate of fetal death in euploid fetuses is only 1 to 2% and consequently the risk for trisomies decreases with gestation. The estimated risks for fetal trisomies 21, 18 and 13 for a woman aged 20 years at 12 weeks of gestation are about 1 in 1000, 1 in 2500 and 1 in 8000, respectively, and the risks of such woman delivering an affected baby at term are 1 in 1500, 1 in 18 000 and 1 in 42 000, respectively. The respective risks for these aneuploidies for a woman aged 35 years at 12 weeks of gestation are about 1 in 250, 1 in 600 and 1 in 1800, and the risks of delivering an affected baby at term are 1 in 350, 1 in 4000 and 1 in 10 000.
Turner syndrome is unrelated to maternal age and the prevalence is about 1 in 1500 at 12 weeks and 1 in 4000 at 40 weeks. For the other sex chromosome abnormalities (47,XXX, 47,XXY and 47,XYY), there is no significant change with maternal age, and because the rate of fetal death is not higher than in euploid fetuses the overall prevalence (about 1 in 500) does not decrease with gestation. Triploidy is unrelated to maternal age and the prevalence is about 1 in 2000 at 12 weeks, but it is rarely seen in live births because most affected fetuses die by 20 weeks.
In the early 1970s, about 5% of pregnant women were aged 35 years or more, and this group contained about 30% of the total number of fetuses with trisomy 21. Therefore, screening on the basis of maternal age, with a cut-off of 35 years to define the high-risk group, was associated with a 5% screen-positive rate (also referred to as false-positive rate, because the vast majority of fetuses in this group are normal) and a detection rate of 30%. In the subsequent years, in developed countries there was an overall tendency for women to get pregnant at an older age, so that now about 20% of pregnant women are 35 years or older and this group contains about 50% of the total number of fetuses with trisomy 21.
MATERNAL SERUM BIOCHEMISTRY
Pregnancies with fetal aneuploidies are associated with altered maternal serum concentrations of various feto-placental products, including AFP, free β-hCG, inhibin A and unconjugated estriol (uE3) and PAPP-A (Merkatz et al., 1984; Canick et al., 1988; Macri et al., 1990; Van Lith et al., 1993; Brambati et al., 1993; Aitken et al., 1996).
In screening using maternal serum biochemical markers, the measured concentration of the markers is converted into a multiple of the median (MOM) of unaffected pregnancies at the same gestation. The Gaussian distributions of log10 (MoM) in trisomy 21 and unaffected pregnancies are then derived, and the ratio of the heights of the distributions at a particular MoM, which is the likelihood ratio for trisomy 21, is used to modify the apriori maternal age-related risk to derive the patient-specific risk.
Early attempts at incorporating maternal serum markers into screening for aneuploidies focused on the second trimester of pregnancy and demonstrated a substantial improvement in detection rates of trisomy 21, compared with screening by maternal age. At a false-positive rate of 5%, the detection rate improves from 30% in screening by maternal age alone to 60 to 65% by combining maternal age with serum AFP and free β-hCG (double test), 65 to 70% with the addition of uE3 (triple test) and 70 to 75% with the addition of inhibin A (quadruple test) (Cuckle et al., 2005; Cuckle and Benn, 2009; Wald et al., 2003a, 2003b). If intact hCG rather than free β-hCG is used, the detection rates are reduced by about 5%.
In the last decade biochemical testing has moved to the first trimester because when this is combined with the ultrasound marker of fetal NT thickness, the performance of screening is superior to second-trimester screening. In trisomy 21 pregnancies, the maternal serum concentration of free β-hCG is about twice as high and PAPP-A is reduced to half compared with euploid pregnancies (Table 2). The measured serum concentrations of these placental products are affected by maternal characteristics, including racial origin, weight, smoking and method of conception as well as the machine and reagents used for the analysis. Consequently, in the calculation of risk for aneuploidies using these products it is necessary to take into account the effects of these maternal variables in defining MoMs before comparing affected and unaffected pregnancies (Kagan et al., 2008a). In euploid pregnancies, the average adjusted value for both free β-hCG and PAPP-A is 1.0 MoM at all gestations, whereas in trisomy 21 the average free β-hCG is 2.0 MoM and the average PAPP-A is 0.5 MoM and they both increase with gestation.
Table 2. Biochemical and sonographic features of trisomies 21, 18 and 13
|NT mixture model|| || || || |
|CRL-independent distribution, %||5||95||70||85|
|Median CRL-independent NT, mm||2.0||3.4||5.5||4.0|
|Median serum free β-hCG, MoM||1.0||2.0||0.2||0.5|
|Median serum PAPP-A, MoM||1.0||0.5||0.2||0.3|
|Absent nasal bone, %||2.5||60||53||45|
|Tricuspid regurgitation, %||1.0||55||33||30|
|Ductus venosus reversed a-wave, %||3.0||66||58||55|
In screening for trisomy 21 by maternal age and serum free β-hCG and PAPP-A, the detection rate is about 65% for a false-positive rate of 5%. The performance is better at 9 to 10 weeks than at 13 weeks because the difference in PAPP-A between trisomic and euploid pregnancies is greater in earlier gestations (Cuckle and van Lith, 1999; Spencer et al., 2003a; Kagan et al., 2008a; Wright et al., 2010). Although the difference in free β-hCG between trisomic and euploid pregnancies increases with gestation, the magnitude of the difference is smaller than that of the opposite relation of PAPP-A.
In trisomies 18 and 13, maternal serum free β-hCG and PAPP-A are decreased (Tul et al., 1999; Spencer et al., 2000a). In cases of sex chromosomal anomalies, maternal serum free β-hCG is normal and PAPP-A is low (Spencer et al., 2000b). In diandric triploidy, maternal serum free β-hCG is greatly increased, whereas PAPP-A is mildly decreased (Spencer et al., 2000c). Digynic triploidy is associated with markedly decreased maternal serum free β-hCG and PAPP-A.
A new biochemical marker of aneuploidies is ADAM12 (A disintegrin and metalloprotease) because in trisomic pregnancies maternal serum levels during the first trimester are lower than in euploid pregnancies (Christiansen et al., 2010). However, it is unlikely that this marker will improve screening, because the reduction is small and there is a significant association between ADAM12 and the traditional biochemical markers of free β-hCG and PAPP-A (Poon et al., 2009).
SCREENING BY FETAL NT THICKNESS
In 1866, Langdon Down reported that in individuals with trisomy 21 (the condition that came to bear his name), the skin appears to be too large for their body (Langdon Down, 1866). In the 1990s, it was realized that this excess skin may be the consequence of excessive accumulation of subcutaneous fluid behind the fetal neck which could be visualized by ultrasonography as increased NT in the third month of intrauterine life (Nicolaides et al., 1992a).
Extensive research in the last 20 years has established that the measurement of fetal NT thickness provides effective and early screening for trisomy 21 and other major aneuploidies (Snijders et al., 1998; Wald et al., 2003a; Nicolaides, 2004; Malone et al., 2005). Furthermore, high NT is associated with cardiac defects and a wide range of other fetal malformations and genetic syndromes (Hyett et al., 1996a; Souka et al., 1998, 2005).
The optimal gestational age for measurement of fetal NT is 11 to 13 weeks and 6 days. The minimum fetal crown–rump length (CRL) should be 45 mm and the maximum 84 mm. The lower limit is selected to allow the sonographic diagnosis of many major fetal abnormalities, which would have otherwise been missed, and the upper limit is such to provide women with affected fetuses the option of an earlier and safer form of termination. Fetal NT can be measured either by transabdominal or transvaginal sonography and the results are similar.
The ability to achieve a reliable measurement of NT is dependent on appropriate training of sonographers, adherence to a standard ultrasound technique in order to achieve uniformity of results among different operators. The magnification of the image should be high so that only the fetal head and upper thorax are included in the picture. A good sagittal section of the fetus in the neutral position should be obtained, and the maximum thickness of the subcutaneous translucency between the skin and the soft tissue overlying the cervical spine should be measured. The mid-sagittal view of the fetal face is defined by the presence of the echogenic tip of the nose and rectangular shape of the palate anteriorly, the translucent diencephalon in the center and the nuchal membrane posteriorly (Plasencia et al., 2007). Deviations from the exact midline plane result in non-visualization of the tip of the nose and visibility of the zygomatic process of the maxilla.
There are three elements in the assessment of NT that can introduce operator bias and either under- or overestimation of the measurement and consequent increase in the variability of measurements. Firstly, the selection of the exact place behind the fetal neck containing the maximum vertical distance between the nuchal membrane and the edge of the soft tissue overlying the cervical spine because the two lines are not usually parallel, secondly, the selection of the appropriate gain to reduce the thickness of the lines and thirdly, accurate placement of the calipers on the two lines. In order to avoid these problems, a semi-automated method of measuring NT thickness has been developed which has the potential to substantially reduce between- and within-operator variability in measurements of NT from a given image (Moratalla et al., 2010).
Fetal NT increases with CRL and therefore it is essential to take gestation into account when determining whether a given NT thickness is increased. There are essentially two approaches to quantifying the deviation of NT from the normal median. One approach is to subtract the normal median from the NT measurement and to produce a deviation in millimeters referred to as delta NT (Pandya et al., 1995a; Spencer et al., 2003b). The other approach is to divide NT by the normal median to produce a MoM value (Nicolaides et al., 1998). In the calculation of patient-specific risks for trisomy 21, the a priori maternal age-related risk is multiplied by the likelihood ratio for a measured NT, which is the ratio of the heights of distributions of measurements in trisomy 21 and unaffected pregnancies. Recently, a new approach has been proposed for quantifying the deviation in the measured NT from the normal. This is based on the observation that in both aneuploid and euploid pregnancies, fetal NT follows two distributions, one which is CRL dependent and another which is CRL independent (Wright et al., 2008). In this mixture model, the distribution in which NT increases with CRL is observed in about 95% of euploid fetuses, 5% with trisomy 21, 30% with trisomy 18, 15% with trisomy 13 and 10% with Turner syndrome. The median CRL-independent NT was 2.0 mm for the euploid group and 3.4, 5.5, 4.0 and 7.8 mm for trisomies 21, 18, 13 and Turner syndrome, respectively.
Several prospective interventional studies in hundreds of thousands of pregnancies have demonstrated that firstly, fetal NT is successfully measured in more than 99% of cases, secondly, the risk of chromosomal abnormalities increases with both maternal age and fetal NT thickness and thirdly, in pregnancies with low fetal NT the maternal age-related risk is decreased. For a 5% false-positive rate, fetal NT screening identifies 75 to 80% of fetuses with trisomy 21 and other major aneuploidies (Nicolaides, 2004).
SCREENING BY FETAL NT THICKNESS AND SERUM BIOCHEMISTRY
There is no significant association between fetal NT and maternal serum free β-hCG or PAPP-A in either trisomy 21 or euploid pregnancies, and therefore the ultrasononographic and biochemical markers can be combined to provide more effective screening than either method individually (Brizot et al., 1994, 1995; Noble et al., 1995; Spencer et al., 1999).
Several prospective interventional studies in many thousands of pregnancies have demonstrated that for a 5% false-positive rate, the first-trimester combined test identifies about 90% of trisomy 21 pregnancies (Krantz et al., 2000; Bindra et al., 2002; Schuchter et al., 2002; Spencer et al., 2003c; Wapner et al., 2003; Nicolaides et al., 2005; Ekelund et al., 2008; Kagan et al., 2009a; Leung et al., 2009).
Timing of ultrasound and blood testing within the first trimester
One option in first-trimester combined screening for trisomy 21 is to perform biochemical and ultrasonographic testing as well as to counsel women in one-stop clinics for assessment of risk (OSCAR) (Bindra et al., 2002; Spencer et al., 2000d). This has been made possible by the introduction of biochemical analyzers which provide automated, precise and reproducible measurements within 30 min of obtaining a blood sample. The ideal gestation for OSCAR is 12 weeks because the aim of the first-trimester scan is not just to screen for trisomy 21 but also to diagnose an increasing number of fetal malformations, and in this respect the ability to visualize fetal anatomy is best at 12 weeks (Souka et al., 2004). The detection rate of trisomy 21 with OSCAR at 12 weeks is about 90% at a false-positive rate of 5%.
An alternative strategy for first-trimester combined screening is for biochemical testing and ultrasound scanning to be carried out in two separate visits, with the first done at 9 to 10 weeks and the second at 12 weeks (Borrell et al., 2004; Kagan, 2008b; Kirkegaard et al., 2008; Wright et al., 2010). It has been estimated that this approach would improve the detection rate from 90% to 93 to 94%. A third option would be to perform the scan at 12 weeks and optimize the performance of biochemical testing by measuring PAPP-A at 9 weeks and free β-hCG at the time of the scan at 12 weeks or even later with an estimated detection rate of 95%. The cost and patient acceptability of the alternative policies of first trimester testing will depend on the existing infrastructure of antenatal care. The potential advantage of two- or three-stage screening in terms of detection rate may be eroded by the likely increased non-compliance with the additional steps.
Additional first-trimester sonographic markers
In addition to NT, other highly sensitive and specific first-trimester sonographic markers of trisomy 21 are absence of the nasal bone, increased impedance to flow in the ductus venosus and tricuspid regurgitation (Table 2). Absence of the nasal bone, reversed a-wave in the ductus venosus and tricuspid regurgitation are observed in about 60, 66 and 55% of fetuses with trisomy 21 and in 2.5, 3.0 and 1.0%, respectively, of euploid fetuses. (Matias et al., 1998; Cicero et al., 2001, 2006; Huggon et al., 2003; Nicolaides, 2004; Faiola et al., 2005; Falcon et al., 2006; Kagan et al., 2009b, 2009c; Maiz et al., 2009).
Assessment of each of these ultrasound markers can be incorporated into first-trimester combined screening by maternal age, fetal NT and serum free β-hCG and PAPP-A resulting in improvement of the performance of screening with an increase in detection rate to 93 to 96% and a decrease in false-positive rate to 2.5% (Kagan et al., 2009b, 2009c; Maiz et al., 2009). A similar performance of screening can be achieved by a contingent policy in which first-stage screening by maternal age, fetal NT and serum free β-hCG and PAPP-A is offered to all cases. Patients with a risk of 1 in 50 or more are considered to be screen positive and those with a risk of less than 1 in 1000 are screen negative. Patients with the intermediate risk of 1 in 51 to 1 in 1000, which constitutes about 15% of the total population, have second-stage screening with nasal bone, ductus venosus or tricuspid blood flow which modifies their first-stage risk. If the adjusted risk is 1 in 100 or more, the patients are considered to be screen positive and those with a risk of less than 1 in 100 are screen negative.
A recently described first-trimester sonographic marker of trisomy 21 is increased flow in the fetal hepatic artery (Bilardo et al., 2010; Zvanca et al., 2011). This marker is also likely to find an application in the assessment of the intermediate risk group after first-stage combined screening.
Individual risk-orientated two-stage screening for trisomy 21 is compatible with the basic principles of clinical practice in all fields of medicine. For example, in the great majority of patients presenting with abdominal pain, the correct diagnosis of the presence or absence of a serious problem is reached after history taking and clinical examination. In a minority of cases, a series of further investigations of increasing sophistication may be necessary before the correct diagnosis is made.
Selective use of ultrasound or biochemistry within the first trimester
The best performance of first-trimester screening is achieved by a combination of maternal age, serum biochemical testing and multiple sonographic markers. At a risk cut-off of 1 in 100, the detection rate of trisomy 21 is about 95% at a false-positive rate of 2.5%. This performance of screening is achieved by either a policy in which biochemical testing is undertaken in all cases or by a contingent policy in which first-stage screening is based on maternal age, fetal NT and either tricuspid or ductus venosus flow, and biochemical testing is then performed in only those with an intermediate risk, which constitute about 20% of the total (Kagan et al., 2010a).
An alternative first-trimester contingent screening policy consists of maternal serum biochemistry in all pregnancies followed by fetal NT only in those with an intermediate risk after biochemical testing. Studies examining the potential performance of such policy have estimated that the detection rates and false-positive rates would be 80 to 90% and 4 to 6%, respectively, and measurement of fetal NT would be necessary in only 20 to 40% of cases (Christiansen et al., 2002; Wright et al., 2004; Vadiveloo et al., 2009; Kagan et al., 2010a; Sahota et al., 2010). The advantage of biochemical testing as a first-stage policy relies on its apparent simplicity. However, interpretation of biochemical results necessitates accurate ultrasonographic measurement of fetal CRL and therefore an ultrasound examination cannot be avoided. In the studies estimating the potential performance of contingent screening, the fetal CRL was measured by appropriately trained sonographers during assessment of fetal NT. It would be wrong to assume that the motivation of sonographers and the accuracy in measuring CRL would remain as high if the scans were carried out purely for measurement of CRL and not examining the fetus.
Major advantages of choosing ultrasound assessment rather than biochemical testing as a first-stage policy are that firstly, there is a substantial reduction in the cost of screening because measurement of maternal serum free β-hCG and PAPP-A is undertaken in only 20% rather than all pregnancies, secondly, the fetal anatomy can be examined leading to early diagnosis of many major abnormalities in all pregnancies rather than in just the subgroup with positive first-stage screening results, thirdly, the Doppler studies can be carried out in the same ultrasound examination as for measurement of fetal NT and fourthly, reversed a-wave in the ductus venosus or tricuspid regurgitation are not only useful in screening for trisomy 21 and other major aneuploidies, but also they can identify pregnancies at increased risk for cardiac defects and adverse pregnancy outcome. The disadvantage is that Doppler assessment of tricuspid and ductus venosus flow can be time consuming and requires appropriately trained sonographers.
First-trimester screening followed by second-trimester scan
In the second trimester scan, each chromosomal defect has its own syndromal pattern of detectable abnormalities (Nicolaides et al., 1992b; Nicolaides, 1996). For example, trisomy 21 is associated with cardiac defects, duodenal atresia, nasal hypoplasia, increased nuchal fold and prenasal thickness, intracardiac echogenic foci, and echogenic bowel, mild hydronephrosis, shortening of the femur and more so of the humerus, sandal gap and mid-phalanx hypoplasia of the fifth finger. In trisomy 18, common findings include strawberry-shaped head, choroid plexus cysts, absent corpus callosum, enlarged cisterna magna, facial cleft, micrognathia, nuchal edema, heart defects, diaphragmatic hernia, esophageal atresia, exomphalos, single umbilical artery, renal abnormalities, echogenic bowel, myelomeningocoele, growth restriction and shortening of the limbs, radial aplasia, overlapping fingers and talipes or rocker bottom feet. Trisomy 13 is associated with holoprosencephaly, microcephaly, facial abnormalities, cardiac abnormalities, enlarged and echogenic kidneys, exomphalos and post axial polydactyly.
If the second-trimester scan demonstrates major abnormalities, it is advisable to offer fetal karyotyping, even if these abnormalities are apparently isolated. The prevalence of such abnormalities is low and therefore the cost implications are small. If the abnormalities are either lethal or they are associated with severe handicap, such as holoprosencephaly, fetal karyotyping constitutes one of a series of investigations to determine the possible cause and thus the risk of recurrence. If the abnormality is potentially correctable by intrauterine or postnatal surgery, such as diaphragmatic hernia, it may be logical to exclude an underlying chromosomal defect—especially because, for many of these conditions, the usual defect is trisomy 18 or 13.
Minor fetal abnormalities or soft markers are common and they are not usually associated with any handicap, unless there is an underlying chromosomal defect. Routine karyotyping of all pregnancies with these markers would have major implications, both in terms of miscarriage and in economic costs. It is best to base counseling on an individual estimated risk for a chromosomal defect, rather than the arbitrary advice that invasive testing is recommended because the risk is ‘high’. The individual risk can be derived by multiplying the a priori risk (based on the results of previous screening by NT and/or biochemistry in the current pregnancy) by the likelihood ratio of the specific abnormality or marker (Benacerraf et al., 1992; Vintzileos and Egan, 1995, Vintzileos et al., 1996; Bahado-Singh et al., 1998; Nyberg et al., 2001; Smith-Bindman et al., 2001; Bromley et al., 2002; Nicolaides, 2003). It has been estimated that the second-trimester scan can improve the detection rate of trisomy 21 achieved by first-trimester combined screening by about 6% for an additional 1.2% false-positive rate (Krantz et al., 2007).
First-trimester screening followed by second-trimester biochemical testing
Three mathematical models have been proposed for the additional use of second-trimester biochemical testing with the aim of improving first-trimester combined screening. Firstly, the integrated test in which all patients have first-trimester NT and PAPP-A and second-trimester AFP, uE3, free β-hCG and inhibin, and the combined results are given on completion of this process so that high-risk patients have second-trimester amniocentesis (Wald et al., 1999). Secondly, step-wise sequential screening, in which all patients have first-trimester NT and serum PAPP-A and free β-hCG, and high-risk patients are offered CVS, whereas low- or intermediate-risk patients have second-trimester AFP, uE3, free β-hCG and inhibin, and if the combined risk from first- and second-trimester testing becomes high, the patients have second-trimester amniocentesis. Thirdly, contingent screening, which is similar to step-wise sequential screening, but second-trimester biochemical testing is performed only in those with an intermediate risk after first-trimester screening (Wright et al., 2004).
The estimated performance of the three approaches is similar with a detection rate of 90 to 94% at a false-positive rate of 5%. The advantages of the contingent approach are that firstly, second trimester testing is avoided in 75 to 80% of patients and secondly, the diagnosis of about 60% of fetuses with aneuploidies is made in the first trimester (Wright et al., 2004, Benn et al., 2005; Cuckle et al., 2008).
The disadvantages of this across trimesters approach to screening are firstly, the performance of screening is poorer that with an integrated first-trimester approach incorporating the new sonographic markers, secondly, reassurance of parents with a low-risk result is delayed by several weeks, thirdly, many women with an affected pregnancy are deprived of the option of safer first-trimester termination of pregnancy and fourthly, many women who do not complete the two-stage test are essentially deprived of screening.
Screening for aneuploidies other than trisomy 21
A beneficial consequence of screening for trisomy 21 is the early diagnosis of trisomies 18 and 13, which are the second and third most common chromosomal abnormalities. At 11 to 13 weeks, the relative prevalences of trisomies 18 and 13 to trisomy 21 are one to three and one to seven, respectively. All three trisomies are associated with increased maternal age, increased fetal NT and decreased maternal serum PAPP-A, but in trisomy 21 serum free β-hCG is increased, whereas in trisomies 18 and 13 this is decreased. In addition, trisomy 13, unlike trisomies 21 and 18, is associated with fetal tachycardia, with the heart rate being above the 95th centile of euploid fetuses in 85% of fetuses with trisomy 13 (Hyett et al., 1996b; Liao et al., 2000; Papageorghiou et al., 2006).
Use of the algorithm for trisomy 21 identifies about 75% of fetuses with trisomies 18 and 13. The combined use of the algorithm for trisomy 21 with specific algorithms for trisomies 18 and 13 improves the detection of these aneuploidies to 95% with a small increase in false-positive rate by about 0.1% (Kagan et al., 2008c). Another beneficial consequence of the use of the combined three algorithms is the early identification of about 85% of fetuses with triploidy (Kagan et al., 2008d).
In addition to the measurement of fetal NT, the 11 to 13 weeks scan can identify many major defects, such as holoprosencephaly, exomphalos and megacystis found in about 1 in 1300, 1 in 400 and 1 in 1600 fetuses, respectively. Aneuploidies, mainly trisomies 18 and 13, are observed in about 65% of fetuses with holoprosencephaly, 55% with exomphalos and 30% with megacystis (Kagan et al., 2010b). At 11 to 13 weeks absence of the nasal bone, abnormal flow in the ductus venosus and tricuspid regurgitation are observed in about 50, 55 and 30%, respectively, of fetuses with trisomies 18 and 13 (Kagan et al., 2009b, 2009c; Maiz et al., 2009).
SCREENING IN TWIN PREGNANCIES
In twin pregnancies, effective screening for chromosomal abnormalities is provided by a combination of maternal age and fetal NT thickness (Pandya et al., 1995b; Sebire et al., 1996a, 1996b; Maymon et al., 2001). The performance of screening can be improved by the addition of maternal serum biochemistry, but appropriate adjustments are needed for chorionicity (Sepulveda, et al., 1996). In dichorionic twins at 11 to 13 weeks, the levels of maternal serum free β-hCG and PAPP-A are about twice as high as in singleton pregnancies, but in monochorionic twins the levels are lower than in dichorionic twins (Spencer and Nicolaides, 2000, 2003; Spencer et al., 2008; Linskens et al., 2009).
In dichorionic twins, patient-specific risks for trisomy 21 are calculated for each fetus based on maternal age and fetal NT, and the detection rate (75–80%) and false-positive rate (5% per fetus or 10% per pregnancy) are similar to those in singleton pregnancies (Sebire et al., 1996a). In the calculation of risk for trisomies, it has been assumed that in each pregnancy the measurements of NT for CRL between the two fetuses were independent of each other. However, recent evidence indicates that in euploid dichorionic twins, the measurements of NT in each twin pair are correlated and this correlation is not a simple reflection of the common effect of sonographers (Wøjdemann et al., 2006; Cuckle and Maymon, 2010; Wright et al., 2011). In screening in twins it is therefore necessary to take this correlation into account because it has a substantial impact on the estimated patient-specific risk for trisomies. First-trimester screening allows the possibility of earlier and therefore safer selective fetocide in cases where one fetus is euploid and the other is abnormal (Sebire et al., 1996b). An important advantage of screening by fetal NT is that when there is discordance for a chromosomal abnormality, the presence of a sonographically detectable marker helps to ensure the correct identification of the abnormal twin should the parents choose selective termination.
In monochorionic twin pregnancies, the false-positive rate of NT screening is higher than in dichorionic twins, because increased NT in at least one of the fetuses is an early manifestation of twin-to-twin-transfusion syndrome, as well as a marker of chromosomal abnormalities (Sebire et al., 1997, 2000; Kagan et al., 2007). In the calculation of risk of trisomy 21, the NT of both fetuses should be measured and the average of the two should be considered (Vandecruys et al., 2005).
Effective screening for all major aneuploidies can be achieved in the first trimester of pregnancy with a detection rate of about 95% and a false-positive rate of less than 3%.
This study was supported by a grant from the Fetal Medicine Foundation (Charity No: 1037116).