Aneuploidies

Aneuploidies, the presence of an abnormal number of chromosomes, are conditions associated with significant morbidity and mortality. Aneuploidies affect approximately 1 in 160 live births , although the overall incidence is higher due to natural pregnancy losses and termination of affected pregnancies. The commonest aneuploidies affecting live births are trisomies 21, 18, and 13 and monosomy X. Trisomies describe an extra copy of a particular chromosome and usually occur due to maternal meiotic nondysjunction. Monosomy X describes a missing copy of the X chromosome in females (45X0, Turner syndrome) which also usually occurs due to maternal meiotic nondysjunction. Sex chromosome aneuploidies such as 45XO and 47XXY (Klinefelter syndrome) are not commonly screened for because in the absence of structural anomalies they would not satisfy the criteria for antenatal screening, and there is insufficient data on the accuracy of cfDNA-based testing for this indication . Sex chromosome aneuploidies will therefore not be discussed further in this chapter, but are extensively addressed in Chapter 15 .

Trisomy 21 (T21), Down’s syndrome, is the commonest trisomy compatible with life. It affects 1.08 per 1000 live births in the United Kingdom and is associated with a number of defects which vary between people in their presence and severity. Possible features include cardiac anomalies, duodenal atresia, mild-to-moderate learning difficulties, hypothyroidism, hearing and vision disorders, seizures, increased risks of certain cancers, particularly leukemia, and Alzheimer’s disease.

Trisomy 18 (T18), Edwards’ syndrome, affects 1 in 6000 to 1 in 8000 live births . The prevalence at the time of screening, 12 weeks, is 1 in 600 for a 35-year-old woman; between 12 weeks and 40 weeks the fetal death rate is around 80%. Possible features include cardiac defects, renal anomalies, severe learning disabilities, omphalocele, central nervous system defects, breathing and feeding difficulties, and physical deformities such as “rocker-bottom” feet, “clenched” hands, micrognathia, and low-set ears. The majority of live-born babies die in the first few days or weeks of life and less than 10% survive to 1 year of age .

Trisomy 13 (T13), Patau’s syndrome, affects approximately 1 in 8000 to 1 in 12,000 live births. The prevalence at the time of screening, 12 weeks, is 1 in 1800 for a 35-year-old woman; again, there is a fetal death rate between 12 and 40 weeks of around 80%. Possible features include cardiac defects, renal anomalies, severe learning disabilities, omphalocele, microcephaly, holoprosencephaly, deafness, seizures, and cleft lip and palate. As with trisomy 18, the vast majority of fetuses die in utero, the majority of live-born babies die in the first few days or weeks of life and less than 10% survive to 1 year of age [5].

Screening by Maternal Age

The rate of autosomal aneuploidies increases significantly with advancing maternal age; the risk of trisomy 21 at 12 weeks of gestation rises from 1 in 1000 for a woman aged 20 years to 1 in 250 for a woman aged 35 years . Similarly, the risk of trisomy 18 increases from 1 in 2500 to 1 in 600 and the risk of trisomy 13 increases from 1 in 8000 to 1 in 1800, over the same time period. Prior to the advent of detailed antenatal ultrasonography and maternal serum biochemistry, maternal age was used as a screening tool for the detection of trisomies. However, maternal age is a poor screening test for trisomies as the majority of babies affected are born to women under the age of 35 years. During the 1970s and 1980s, the estimated detection rate with maternal age as a screening tool was 30% ( Fig. 1 ).

Maternal age and increasing aneuploidy risk [ ].

From thesis P. Pandya with permission.

Once identified as “high risk” for trisomies by maternal age, the diagnostic option available to the mother was invasive testing, which less than half of women at the time opted for. Amniocentesis in the 1980s was estimated to carry a 1.0% additional risk of spontaneous pregnancy loss . As the vast majority of pregnancies conceived by women over the age of 35 years are not affected by trisomies, using maternal age alone as a screening tool to offer amniocentesis would therefore lead to a high loss of nonaffected fetuses with a low detection rate, therefore it is no longer recommended.

Screening by Maternal Biochemistry

Measurement of maternal serum biochemistry was initially intended to screen for neural tube defects; however, it was shown that alpha-fetoprotein (AFP) was low in fetuses with trisomy 21, leading to further research in this area. Serum biochemistry was initially suggested as a screening option for women under 35 years of age but, as it was increasingly shown to be more sensitive than maternal age alone, it also became an option for women over 35 years of age . As several maternal biochemical blood markers are affected by trisomic pregnancies, these can be measured and converted to a gestational age-specific multiple of the median (MoM), which is compared to a level at which one would expect an unaffected pregnancy. Markers investigated during the 1980s and 1990s for second trimester screening included AFP, human chorionic gonadotropin (hCG), unconjugated estriol and inhibin A.

Trisomy 21 is typically associated with low levels of AFP and estriol, and high levels of hCG and inhibin A. Adding the results of these biomarkers to maternal age increases the sensitivity of second trimester screening in a stepwise fashion, as follows :

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  Double test (AFP and hCG): 55%–60% detection, 5% false positive rate.

  
  
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  Triple test (AFP, hCG and E3): 60%–65% detection, 5% false positive rate.

  
  
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  Quadruple test (AFP, hCG, inhibin A, unconjugated estriol): 65%–70% detection, 5% false positive.

Using free instead of total hCG increases the maximum detection rate by about 5% in all the earlier tests.

In the first trimester, hCG and PAPP-A can be used for serum screening. Trisomy 21 is typically associated with high levels of hCG, as in the second trimester, but low levels of PAPP-A. Combining maternal age with hCG and PAPP-A levels in the first trimester gives a detection rate of approximately 65% with a false positive rate of 5% . This detection rate decreases as the first trimester progresses, as PAPP-A is a more powerful marker at earlier gestations.

Screening by Ultrasound

Maternal serum biochemistry alters with gestational age and calculation of the MoM requires accurate pregnancy dating. Ultrasound was initially introduced to date the pregnancy prior to measurement of AFP. In addition, ultrasound scanning equipment, image resolution, and technique were also advancing. In the 1990s, evidence accumulated that increased fluid at the back of the fetal neck (nuchal translucency, NT) ( Fig. 2 ) in the first trimester was associated with fetal trisomies , as well as other chromosomal and structural abnormalities. Screening for trisomy 21 using maternal age and NT together gives a detection rate of 75%, with a false positive rate of 5% . Adding in first trimester maternal serum biochemistry increases the detection rate further, and so the combined test (maternal age, NT, hCG, and PAPP-A) has a detection rate for trisomy 21 of 85%–90% with a 5% false positive rate ( Table 1 ).

Ultrasound image of normal and increased nuchal translucency.

From “Fetal Medicine: Basic Science and Clinical Practice.” Chapter on First Trimester Anomalies; courtesy Mr. Fred Ushakov. Publisher Elsevier, with permission.

Comparisons of first trimester screening by the combined test and second trimester screening by the quadruple test have shown the combined test to be superior. However, performing both sets of screening in a sequential fashion, that is, maternal age, nuchal translucency, and maternal serum biochemistry in both first and second trimesters, may have a higher detection rate (90%–95%) for trisomy 21 . However, sequential screening introduces diagnostic delay and requires a second blood test and has not been widely accepted by women or healthcare professionals.

It is important to note that all the rates discussed previously are for the detection of trisomy 21. Trisomies 18 and 13 are also associated with an increased NT and low PAPP-A in the first trimester but are instead associated with low levels of hCG. The use of the previous combined screening protocol will identify approximately 75% of fetuses with trisomy 18 or 13 when using algorithms for trisomy 21 but over 90% when using trisomy 18/13 algorithms . As in over two-thirds of fetuses with these trisomies, there are associated anatomical defects that can be detected early in pregnancy (e.g. holoprosencephaly, exomphalos), their overall detection rate is likely to be significantly increased with the use of routine first trimester ultrasound scanning.

In the United Kingdom and in many parts of the world, the current trisomy screening recommendations are to use the combined test in the first trimester and, if that results in a low risk (less than 1 in 150 at term), no further invasive testing is performed. For women presenting in the second trimester (between 14 + 2 and 20 + 0 weeks’ gestation), the quadruple test is performed with the same cutoff of 1 in 150. All women are subsequently offered a detailed anomaly scan at 18–20 + 6 weeks of gestation, at which time the detection of fetal anomalies will result in the offer of invasive testing.

Uptake of Screening

Screening for fetal aneuploidy is parental choice and uptake should not be used as a marker of screening performance. Data regarding the uptake of fetal trisomy screening in the United Kingdom is therefore not collected, and so estimates are calculated based on the total number of tests and on the total number of births (60%–74%) or retrospective questionnaire studies (65%). In the United States, screening uptake appears similar (70%), but increases in areas where state law requires all women to be offered screening (80%). A retrospective Canadian study showed similar rates of screening uptake (62.2%) with wide regional variation (27.8%–80.3%), and an Australian study showed a slightly lower uptake of screening (45%). Within Europe, rates of screening uptake also vary widely, from under 30% in the Netherlands to over 90% in Denmark . The availability, cost, and societal perspective of screening is probably responsible for this wide variation in uptake.

Diagnostic Tests

Invasive testing for trisomies by amniocentesis or chorionic villus sampling (CVS) is diagnostic and not a screening test. It is offered in place of screening in many countries either if the risk is already judged to be high (e.g., ultrasound abnormalities, balanced parental translocations, previously affected pregnancies) or for maternal choice . As discussed previously, invasive testing is traditionally quoted to carry a 1.0% risk of additional spontaneous pregnancy loss, based on a large randomized trial of amniocentesis in 1986, in which the actual pregnancy loss rate was 1.7% in the study group and 0.7% in controls . Amniocentesis prior to 15 weeks’ gestation has been associated with a higher rate of fetal loss and talipes and is therefore not recommended. CVS prior to 10 weeks’ gestation has been associated with limb hypoplasia and is also not recommended.

Observational studies in the early 2000s suggested the fetal loss rate following second trimester amniocentesis to be lower than previously thought . Rates of fetal loss following CVS were quoted as 1.9% in 2009 . More recent publications have suggested that fetal loss following amniocentesis and CVS to be lower than previously described. A 2015 study found a procedure-related risk for amniocentesis of 0.11%, with an actual pregnancy loss rate of 0.81% in the study group and 0.67% in controls, and a procedure-related risk for CVS of 0.22%, with an actual pregnancy loss rate of 2.18% in the study group and 1.79% in controls. In 2016, a population-based study of nearly 150,000 women found no increased risk of fetal loss with either amniocentesis or CVS.

CELL-FREE DNA NIPT Use in Aneuploidy Screening

Since 2010, the use of NIPT for aneuploidy screening has been explored extensively. The reader is referred to Chapter 3 of this book for discussion on methodology of cfDNA NIPT use in this situation and to Chapter 6 for discussion of cfDNA NIPT use in twin pregnancies.

The most recent meta-analysis included 35 studies on cfDNA NIPT use in aneuploidy screening and only included studies in which data on pregnancy outcome were provided for more than 85% of the study population. It found a 99.7% detection rate for trisomy 21, with a false positive rate of 0.04%. This compares favorably with the combined screening detection rate of 85%–90% and false positive rate of 5%, as described previously. In this same meta-analysis, trisomy 18 was found to have a detection rate of 97.9%, with a false positive rate of 0.04% and trisomy 13 was found to have a detection rate of 99.0%, with a false positive rate of 0.04%. This also compares favorably with standard combined screening ( Table 2 ).

Despite these high detection rates and low false positive rates, it is important to emphasize that cfDNA NIPT is still a screening test and that women with a high risk result require invasive testing to confirm the findings. cfDNA NIPT is not considered diagnostic as the “fetal DNA” detected is actually placental in origin; it originates from apoptosis of placental cytotrophoblast and syncytiotrophoblast cells . In cases of placental mosaicism, the cytotrophoblast may contain an abnormal cell line which is not present in the fetus. Although CVS also obtains placental tissue, this is considered diagnostic when it analyses both the cytotrophoblast and the mesenchyme, which is more likely to represent the fetal karyotype . Additionally, a “vanishing twin,” maternal chromosome abnormalities, and maternal (malignant) disease may all affect the cell-free DNA in maternal plasma and suggest an abnormality which is not fetal in origin. For more information, the reader is referred to Chapter 5 of this book.

cfDNA NIPT used for aneuploidy screening may fail to provide a result in some cases; this is most commonly due to a low fetal fraction, which itself may be due to a number of reasons. In the aforementioned meta-analysis , the reported failure rates ranged from 0% to 12.2%. A study investigating failed results found that fetal fraction was reduced with rising maternal body mass index, increased maternal age, South Asian racial origin compared to Caucasian origin, and in assisted conceptions. They found a failure rate of 2.9% in unaffected pregnancies, 1.9% in trisomy 21, 8.0% in trisomy 18, and 6.3% in trisomy 13. In trisomies 18 and 13 there is a relatively small placenta, reflected by low PAPP-A levels, and this may be one of the reasons for cfDNA NIPT failure. This reinforces the central role of a detailed first trimester scan assessing fetal anatomy and specifically looking for major anomalies that may be associated with these trisomies.

NIPT Use in High-Risk Populations

High-risk populations have been described as women with an increased risk following conventional screening (combined or quadruple test). Different studies have used different cutoffs to offer cfDNA NIPT in a contingent model of screening. In the meta-analysis described previously, 30 of the 35 studies were in high-risk pregnancies. The authors found no difference in screening performance between high-risk and routine populations in subgroup analysis.

Two major studies have investigated the use of NIPT in high-risk populations, with the offer of NIPT contingent on results from standard screening.

In the first , over 11,000 pregnancies in two UK hospitals were included. Women underwent routine combined screening in the first trimester; those with a risk greater than 1 in 100 at 12 weeks (equivalent to 1 in 150 live birth risk) (high risk) were offered invasive testing, NIPT or no further testing, and those with a risk between 1 in 101 and 1 in 2500 (intermediate risk) were offered cfDNA NIPT or no further testing. There were 47 fetuses with trisomy 21 in this study; 41 (87%) of them were in the high-risk group, 5 cases were in the intermediate-risk group, and 1 case was in the low-risk group. Of the 24 fetuses with trisomy 18, 22 (92%) were in the high-risk group and 2 were in the intermediate-risk group. All 4 (100%) of the fetuses with trisomy 13 were in the high-risk group. NIPT failed to provide a result after first sampling in 2.7% of cases. In 63.0% of those retested, the second test provided a result. cfDNA NIPT correctly identified 97.7% of cases of trisomy 21 tested for (43/44), with one false negative case. cfDNA NIPT correctly identified 87.5% (21/24) of trisomy 18 cases tested for and 50% (2/4) of trisomy 13 cases tested for ( Table 3 ).

Not all patients opted for further investigations after initial screening. In the high-risk group, 37.6% (173/460) opted for invasive testing, 60.0% (276/460) for cfDNA NIPT, and 2.4% (11/460) declined further investigation. In the intermediate-risk group, 91.5% (3249/3552) opted for cfDNA NIPT and 8.5% (303/3552) declined further investigations. Overall, taking into account test performance and patient choice, contingent screening detected 91.5% (43/47) cases of trisomy 21, and 100% (28/28) cases of trisomy 18 or 13. NIPT was associated with a 43% reduction in invasive testing in the high-risk group, from 65.6% performed the previous year in high-risk patients to 37.6% in this study. Patients were more likely to opt for invasive testing with increasing risk of trisomies and increasing nuchal translucency and were more likely to opt against invasive testing if they were of Afro-Caribbean origin or given the combined screening results on a different visit to having the NT ultrasound scan (i.e., their unit did not offer a “one-stop” service). In the intermediate-risk group, 91.5% (3249/3552) opted for cfDNA NIPT and 8.5% (303/3553) had no further investigations. The option of cfDNA NIPT therefore increased the number of women who accepted any form of further testing (cfDNA NIPT or Invasive Prenatal Diagnosis). Therefore while one may argue that offering cfDNA NIPT in a contingent model may reduce detection rate, in reality because more women will have testing the overall detection rate will increase. Termination of pregnancy following prenatal diagnosis of trisomy 21 was 74% overall. For women in the high-risk group, the rate of termination was 92.6% in those who chose invasive testing which was subsequently positive, and 35.7% in those who chose cfDNA NIPT and subsequently received a high-risk result (9/13 of these women proceeded to invasive testing, 5/13 had a termination of pregnancy, and 4/13 had no further testing). For women in the intermediate-risk group there were five cases of trisomy 21. In four of these cases the parents had opted for cfDNA NIPT, which gave a high risk result for three of them. Two of these three opted for termination of pregnancy. These figures suggest that women were using cfDNA NIPT for information and preparedness but not necessarily to proceed to termination of pregnancy.

The RAPID (Reliable, Accurate, Prenatal non-Invasive Diagnosis) study , conducted from our institution, investigated the use of cfDNA NIPT in high-risk populations and included over 30,000 patients in eight UK maternity units. Women underwent routine combined or quadruple testing depending on gestational age; those with a risk greater than 1 in 150 were offered invasive testing, NIPT or no further testing, and those with a risk between 1 in 151 and 1 in 1000 were offered NIPT or no further testing. There were 934 women with a risk of greater than 1 in 150, 17.8% (166/934) opted for invasive testing, 74.4% (695/934) opted for NIPT, and 7.8% (73/934) declined further investigations. Of the women with a risk between 1 in 151 and 1 in 1000, the proportion opting for NIPT was 80.3% (1799/2241). The authors concluded that in England with an estimated 698,500 births per annum, offering NIPT as a contingent test to women with a Down’s syndrome screening risk of at least 1/150 would increase detection by 195 cases (95% uncertainty interval 34–480) with 3368 (95% uncertainty interval 2279–4027) fewer invasive tests and 17 (95% uncertainty interval 7–30) fewer procedure-related miscarriages for a nonsignificant difference in total costs.

NIPT Use in Low-Risk Populations

The studies described previously evaluated cfDNA NIPT in a clinical setting which was contingent on routine screening. The use of cfDNA NIPT as primary screening, without prior routine screening, has been reviewed in several studies, shown in Table 4 .

The data show that cell-free DNA-based NIPT can be used as a primary screening method with improved detection rates, much higher positive predictive value, and reduced false positive rates for trisomy 21 compared to standard screening.

At the moment, the main limiting factors for implementation of cfDNA as primary screening are the cost of testing, and management of cases where NIPT fails to provide a result.

Consequences for Existing Services

Cell-free DNA-based NIPT is clearly a major advance for prenatal screening, and its use has rapidly progressed during this decade. However, it seems unlikely that cfDNA NIPT will entirely replace existing services, such as ultrasound scanning and invasive testing, for several reasons. Firstly, cfDNA NIPT requires an ultrasound scan to be performed for demonstrating viability, dating, and to assess for multiple pregnancy or “vanishing twin,” all of which has an effect on test performance. First trimester scanning also offers an early chance to assess for major structural abnormalities such as anencephaly, holoprosencephaly, and ventral wall defects which cannot be assessed by cfDNA NIPT.

Secondly, the presence of a fetal anomaly detected on ultrasound scanning should prompt the offer of invasive testing directly rather than cfDNA NIPT which, as previously discussed, remains a screening test and is not diagnostic. Trisomies 21, 18, and 13 are not the only abnormalities which are clinically useful to detect; chromosomal microarray analysis has been shown to identify additional, clinically significant information in 1.7% of pregnancies when compared to standard karyotyping when the indication for testing was maternal age or high-risk screening, and in 6.0% of pregnancies when compared to standard karyotyping in the presence of a structural anomaly . A positive cfDNA NIPT will always need to be confirmed with invasive testing, even if the indication is an ultrasound anomaly. Choosing cfDNA NIPT in case of ultrasound anomalies will therefore cause delay in diagnosis and incur extra cost versus going straight to invasive testing.

Thirdly, there will be a small number of women, also discussed previously, who will not receive a result with NIPT (“test failure”). In this situation, the options would be to repeat the test, perform standard combined screening, or proceed straight to invasive testing. This latter option is recommended by the ACOG , due to the increased risk of aneuploidy with a “test failure.” Also, given the ability of invasive testing to assess for chromosomal anomalies other than trisomies, including clinically significant chromosomal copy number variations, and the most recent data quoted earlier in this chapter suggesting that the risk of associated pregnancy loss may be much less in modern practice than previously suggested, it has been argued that all women should be offered diagnostic testing directly without prior screening .

Serum biochemistry may also retain a role in prenatal screening outside of aneuploidy risk assessment; it is well documented that low PAPP-A levels are associated with growth restriction, preterm delivery, hypertensive disorders of pregnancy, and stillbirth. Many first trimester prediction models for preeclampsia or stillbirth include PAPP-A values, and models such as that used in the ASPRE trial have shown that treating women at high risk for preeclampsia with aspirin reduces the incidence of the disease. Several angiogenic markers, such as placental growth factor (PlGF) and soluble fms-like tyrosine kinase 1 (sFlt-1) have also been shown to be associated with an increased risk of developing preeclampsia and may have a role in routine first trimester screening in the future.

However, despite the continued clinical value of ultrasound scanning, invasive testing, and serum biochemistry, it seems likely that increasing use of cfDNA NIPT will lead to a reduction in biochemical testing, and cytogenetic/molecular assessment of fetal cells or tissue. As noted in a recent RCOG Scientific Impact Paper , overall there will be a significant reduction in the number of invasive procedures and this will impact future training of specialists in fetal medicine. It seems likely that invasive procedures will be undertaken in fewer, larger centers than at present.

Cell-Free DNA-Based NIPT Implementation Models, Costs, and Types of Test

The two main methods of implementing cfDNA NIPT screening which have been used to date are as follows:

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  High-risk or contingency screening: offering NIPT to a selected group of women, based on their results from standard screening.

  
  
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  Low-risk or population screening: offering NIPT to all women, in place of standard screening.

The evidence for both these models has been discussed previously in this chapter. It is clear that the detection rate for cfDNA NIPT in contingency screening will depend on the risk level at which cfDNA NIPT is offered and the uptake of testing. It is also very likely that cost will play a role in which of these models is implemented and which cutoff is used in a contingent model. Although the cost of cfDNA NIPT is currently high in many countries, wider introduction and advances in sequencing technology would be expected to decrease expenditure in invasive testing. In the United Kingdom, the RAPID trial previously discussed found that offering cfDNA NIPT to women with a combined screening result of more than 1 in 150 would not have a significant effect on cost. It is likely that costs of cfDNA NIPT will continue to decrease as technology advances and volume increases; it may be that, as this happens, it becomes cost effective to offer at lower risks or on a population basis.

Several commercial companies currently offer cfDNA NIPT screening—this financial drive may well affect implementation via consumer advertising and medical education. It may be the case that the private sector involvement has led to a publication bias in cfDNA NIPT studies. It is difficult to compare the performance of cfDNA NIPT tests from different companies. Two main methods of cfDNA NIPT are currently used: massively parallel shotgun sequencing (MPSS) and targeted sequencing (TS). Single-nucleotide polymorphism (SNP) is a form of targeted sequencing. A recent Cochrane review found no difference in clinical sensitivity or specificity for either method. This review concluded that “NIPT appears sensitive and highly specific for detection of fetal trisomies 21, 18 and 13 in high-risk populations. There is a paucity of data on the accuracy of NIPT as a first-tier screening test in a population of unselected pregnant women.” The Cochrane review included four of the five publications in Table 4 . Cell-free DNA testing at present is not well validated for microdeletions or duplications but this is likely to change in the future with improving technology and increasing the depth of sequencing.

Ethical Issues

When considering the implementation of cfDNA NIPT for aneuploidy screening, it is obvious that a number of ethical and moral issues are raised, which it is important to be cognizant of. The Nuffield Council on Bioethics report explores many of these in depth; we will highlight a few of the main ones here.

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  Harm reduction: as widespread cfDNA NIPT implementation is likely to reduce the rate of invasive testing, it will therefore reduce the number of procedure-related miscarriages (which may be small).

  
  
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  Justice/fairness: many women are already accessing cfDNA NIPT in many countries around the world in the private sector. In some cases, such as the United Kingdom, a few hospitals currently offer cfDNA NIPT in a contingent model in a public health setting whereas other publicly funded hospitals do not. One could argue that making a highly accurate test only available to women who can afford it, or on the basis of postcode, is unfair to others.

  
  
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  Reproductive autonomy: by offering more accurate screening than the current standard, cfDNA NIPT furthers the ability of women and their partners to choose the circumstances of their pregnancy and either make plans for giving birth to a child affected by aneuploidy or make plans for termination of the pregnancy.

  
  
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  Information giving/understanding: as cfDNA NIPT is not associated with a miscarriage risk and is quick and simple to do, it may be that the implications of test outcomes are not fully explained or considered. As the Nuffield Council report discusses: “some healthcare professionals may be focusing on medical problems when imparting information about Down’s syndrome, without describing more fully what it can be like to have a child with Down’s syndrome. The provision of accurate, balanced information that supports all screening choices equally, and the need for sufficient time to discuss any concerns are essential requirements for the introduction of NIPT in the NHS.”

  
  
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  Sex determination: although not the primary aim of aneuploidy screening, cfDNA NIPT (as with invasive karyotyping) is able to determine the sex of the fetus at an early gestation. This may increase the risk of sex selective terminations taking place. We recommend that unless clinically indicated (e.g., suspected congenital adrenal hyperplasia), routine fetal sex determination should not be offered by cfDNA NIPT.

  
  
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  Termination of pregnancy: as the Nuffield Council report discusses, “Introducing cfDNA NIPT in the NHS could lead to an increase in the number of terminations following a diagnosis of Down’s, Edwards’ or Patau’s syndrome. Some believe this amounts to eugenics. If this leads to a significant reduction in the number of people born and living with these syndromes, it is possible that the quality of health and social care they receive and the importance attributed to research into these syndromes will be affected. Making cfDNA NIPT available in the NHS could be perceived as sending negative and hurtful messages about the value of people with the syndromes being tested for.” This issue has been raised as a concern of many forms of screening prior to NIPT including the current combined screening which, as discussed previously, is widely used across the world.

International Implementation of Cell-Free DNA-Based NIPT

In writing this section, we are aware that the international use of cfDNA NIPT is rapidly evolving and likely to change very quickly from what is described as follows.

It has been shown that in the majority of high- and middle-income countries, cfDNA NIPT is available . In the majority of these countries this is through the private sector. In a number of countries, such as Belgium, The Netherlands, Singapore, Australia, Canada, and the United States, cfDNA NIPT can also be available through the public sector. Prices have been quoted as ranging from $350 (Australia) to $2900 (United States), with an average cost of $874 worldwide.

In the United Kingdom, the National Screening Committee recommendation has been that women who are at higher risk (defined as greater than 1 in 150) should be offered cfDNA NIPT as an additional step in the screening pathway along with the options of invasive testing or no further testing. This will come into effect in the public sector in Autumn 2018.

The ACOG Committee Opinion has also recommended a contingency model of NIPT implementation rather than population-based screening.

Early 2017 the Netherlands has implemented primary or population-based screening. As will be the case in the UK, fetal sex is not communicated. Part of the costs are paid by the patient and the costs are equal for first trimester combination test and cfDNA NIPT. Initial experience suggests an increase in uptake from approximately 30% for combined screening to 40% for cfDNA NIPT. Termination of pregnancy rate in women who have a trisomy confirmed remains unchanged compared to combined screening. The Dutch laboratories use massively parallel sequencing as a technology, and perform a 3-year study into the impact of analysis filters on the quality of the cfDNA NIPT test, into the number and type of additional findings and their clinical utility, and their impact on the participants. Participants can elect whether to have additional findings communicated to them or not.

The International Society for Prenatal Diagnosis have stated that the following options are all currently considered appropriate:

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  NIPT as a primary test offered to all pregnant women

  
  
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  NIPT secondary to a high-risk assessment based on serum and ultrasound screening protocols

  
  
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  NIPT contingently offered to a broader group of women ascertained as having high or intermediate risks by conventional screening.

  
  
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  Standard combined (ultrasound nuchal translucency at 11–13 completed weeks combined with serum markers at 9–13 weeks) or quadruple screening, or both (sequential screening).

The International Society of Ultrasound in Obstetrics and Gynaecology (ISUOG) Updated Consensus Statement from 2017 stresses the importance of a first trimester ultrasound scan between 11 and 13 + 6 weeks, regardless of a woman’s intention to undergo cfDNA testing. The statement advises that following a normal early pregnancy scan, as defined by ISUOG guidelines, three options regarding screening or testing for trisomy 21 and, to a lesser extent, trisomies 18 and 13 should be explained, and might be considered for women who wish to have further risk assessment: (1) screening strategies based on individual risk calculated from maternal age and nuchal translucency measurement and/or maternal serum markers and/or other ultrasound markers in the first trimester, if needed and requested followed by cfDNA NIPT or invasive testing. Expert opinion suggests that in women considered at very high (> 1:10) risk after combined screening cfDNA testing should not routinely replace invasive testing (2) cfDNA NIPT as a first-line screening test (3) invasive testing based on a woman’s preference or background risk without risk calculation. ISUOG confirms that experience in low-risk women confirms the high detection rates published for high-risk populations and states that using cfDNA-based NIPT for intermediate or low-risk patients might be endorsed as a widely available option when new data emerge and costs decrease.

The American College of Medical Genetics and Genomics states that “New evidence strongly suggests that NIPS can replace conventional screening for Patau’s, Edwards and Down syndromes across the maternal age spectrum, for a continuum of gestational age beginning at 9 and 10 weeks, and for patients who are not significantly obese.” They further recommend allowing patients to select diagnostic or screening approaches for the detection of genomic changes that are consistent with their personal goals and preferences.