For a woman hoping to start a family, discovering two lines on her urine pregnancy test may initiate profound excitement at a future vision of motherhood. This is followed in the subsequent weeks by a realisation that pregnancy is a potentially difficult journey, with many choices and challenges to navigate. One important decision the woman faces is whether to have first trimester screening to assess her risk of having a baby with Down syndrome. Currently, in the UK, approximately two thirds of all pregnant women opt for Down syndrome screening, with about one third declining the test for various reasons including religious, cultural or personal factors.
Down syndrome is a condition caused by an extra (and so three in total, hence ‘trisomy’) chromosome number 21 in all cells of the body. At least 95% of cases are not inherited, instead occurring due to a chance error in cell division called meiosis, which happens before the time of conception. First meiosis occurs during fetal life in females, and second meiosis occurs with formation of the ovum during each menstrual cycle. Although the risk of Down syndrome increases with advancing maternal age, more babies with Down syndrome are born to younger women (because more babies in total are born to younger women). With more than 37 000 babies born in the UK with Down syndrome in 2011 (Wu and Morris, 2013), this condition is one of the most common causes of disability. However, the clinical characteristics of Down syndrome are variable. Many people with Down syndrome have some degree of learning disability, around 50% have a congenital cardiac anomaly, and early-onset dementia and leukaemia are more likely (Polk et al, 2015). Despite these potential health risks, many people with Down syndrome attend mainstream school during childhood and are in work as adults (Jensen and Bulova, 2014).
Current screening methods
It has been possible to screen for Down syndrome during pregnancy since the 1970s, initially using advanced maternal age (Nadler and Gerbie, 1970). The NHS fetal anomaly screening programme (FASP) sets national standards and evidence-based guidance on methods of screening. The current practice is to offer first trimester combined screening (FTCS) to all pregnant women, which includes an ultrasound measurement of the nuchal translucency (the fluid at the back of the fetus's neck), and measurement of two placentally derived protein molecules (beta human chorionic gonadotrophin (βhCG) and pregnancy-associated placental protein-A (PAPP-A)) in the maternal serum and maternal age. The FASP standards for FTCS are a detection rate of 85% for a false positive rate of 2% (NHS Screening Programmes, 2015). If a woman presents too late for FTCS, Down syndrome screening between 14+2 weeks and 20+0 weeks is still offered, but by a quadruple test. This blood test measures the concentrations of four molecules (alpha-fetoprotein, βhCG, inhibin-A and unconjugated oestriol) to determine, with the maternal age, the chance of Down syndrome. A quadruple test is not as accurate as a FTCS, with a detection rate of 80% and a false positive rate of 3.5%. In the FTCS, the chances of Down syndrome based on the combination of the nuchal translucency, serum markers and the maternal age is then generated for the woman's pregnancy, with a cut-off of 1: 150 marking the distinction between a low-risk and higher-risk result. Currently, women with a higher-risk result (≥ 1: 150) are offered a diagnostic invasive test—either chorionic villus sampling from 11+0 weeks or amniocentesis from 15+0 weeks' gestation.
‘The first trimester combined screening detection rate is only up to 90%, so every year some affected babies are born in the UK after a low-risk screening result has been given’
Because FTCS comprises both ultrasound and biochemical analysis, it has the advantage that it can include a basic fetal anatomy check and determination of the number of fetuses. In addition, a low PAPP-A result has been suggested as a screen for fetal growth restriction. However, there are two major disadvantages of the current FASP-recommended screening methods: the FTCS detection rate is only up to 90%, so every year some affected babies are born in the UK after a low-risk first or second trimester screening result has been given; and the invasive diagnostic tests (the vast majority being done in unaffected pregnancies) carry a miscarriage rate of about 1%.
Non-invasive prenatal testing
The presence of cell-free fetal DNA (cffDNA) in maternal circulation was discovered by Lo et al (1997). It is now known that fetal DNA is reliably detectable in maternal circulation from 10 weeks' gestation, and advances in fetal DNA sequencing technologies have allowed detection of the extra amount of fetal DNA fragments from particular chromosomes as occurs in the trisomies 21, 13 and 18. The word ‘testing’ has been used rather than ‘screening’ because of its increased reliability, but instead of ‘diagnosis’ because of the need for invasive confirmation of positive results.
Advantages
With highly accurate detection (99.2%) and low false positive rates (0.09%) for trisomy 21, non-invasive prenatal testing (NIPT) is a far superior screening method to FTCS. It is also highly accurate in the detection of the other common trisomies 18 (Edwards' syndrome) and 13 (Patau's syndrome), with detection rates of 96.3% and 91%, respectively (Gil et al, 2015). Being a non-invasive screening test, it is associated with a 66% reduction in the number of miscarriages occurring secondary to invasive testing (Garfield and Armstrong, 2012) and so is likely to be a more attractive screening option for pregnant women. The general population in the UK who are currently offered FTCS are predominantly low risk. Performance of NIPT for Down syndrome in low (or ‘all’) risk populations has been addressed by a prospective multicentre study of more than 15 000 low-risk pregnancies, which showed that non-invasive fetal DNA analysis for the detection of trisomy was far superior to standard first-trimester screening, with a detection rate of 100% for a false positive rate of 0.06% (Norton et al, 2015).
‘The increased detection rate and reduced miscarriage rate associated with non-invasive prenatal testing compared to previous aneuploidy screening and diagnosis means it is becoming a reality in standard clinical practice’
The use of fetal DNA detection in maternal plasma has become standard NHS practice for a variety of conditions, such as red cell alloimmunisation (Daniels et al, 2004), and for routine care of rhesus-negative women (Bills and Soothill, 2014; Ford and Soothill, 2016). This demonstrates that the ease of blood sampling for NIPT for some conditions is already performed within the NHS.
Disadvantages
NIPT is not without its disadvantages. It is associated with a ‘no result’ rate, where the test does not give a result in up to 10% of pregnancies. In addition, because maternal obesity is associated with an increased circulating blood volume (which dilutes the concentration of fetal DNA in maternal blood), this failure rate is higher in obese women. There are also case reports of discrepant results between the NIPT and the fetal genetic testing, thought to be due to placental mosaicism, ‘vanished’ twins and maternal malignancies (Pan et al, 2014). Another consequence is that, because NIPT reduces the number of invasive tests, there is a possibility that fetal medicine specialists may become deskilled in invasive techniques, which will still be required for other pathologies such as intrauterine transfusion for fetal Rhesus disease. Finally, because it is a relatively new screening method involving expensive laboratory-based technologies, costs are high and this is an issue for the publicly-funded NHS.
The increased detection rate and reduced miscarriage rate associated with NIPT compared to previous aneuploidy screening and diagnosis means that NIPT is becoming a reality in standard clinical practice. However, the exact method by which it will be used could take one of two forms: primary or secondary contingent screening.
Implementation of NIPT
Primary screening
Primary aneuploidy screening via NIPT would entail an end to serum screening for PAPP-A, βhCG, AFP and oestradiol. Instead, it would involve a first trimester scan to diagnose gestational age, number of fetuses and fetal viability but no nuchal thickness measurement, followed by the offer of venepuncture for cffDNA detection. A high-risk result from NIPT would then necessitate an invasive test to confirm the screening result. This approach would diagnose more cases of trisomy 21 compared to standard screening and fewer babies being born with Down syndrome following a false negative standard screening result (Neyt et al, 2014). It would also result in a significant (approximately sevenfold) reduction in iatrogenic miscarriages as up to 98% of prenatal diagnostic invasive procedures could be avoided due to the far lower false positive rate of NIPT (Dan et al, 2012). Cost-consequences models of primary NIPT have reported elevated costs when compared to standard screening. Neyt et al (2014) reported that the cost per case of trisomy 21 diagnosed might be around € 236 000, a tripling of costs compared to FTCS. However, the cost of NIPT has been falling rapidly over the last few years and may soon meet Neyt et al's (2014) estimate that the cost of the test will need to be lowered by around two thirds to keep the overall cost of aneuploidy screening constant.
Secondary contingent screening
A second method of NIPT implementation is secondary contingent screening; that is, offering NIPT to all women (with a nuchal translucency below 3.5 mm) who receive a high-risk FTCS result. By continuing the FTCS, the ability to screen for fetal growth restriction via PAPP-A concentrations would be preserved, and it is associated with a reduced rate of iatrogenic miscarriage and increased rate of trisomy 21 detection (although on a magnitude less than with primary NIPT). The global opinion is divided regarding the cost-effectiveness of this approach. A study involving 32 000 pregnancies in Australia reported that the costs would increase by 9.7% if the contingent screening approach were to be introduced, but that it would allow an increase in the detection rate of trisomy 21 and an 88% reduction in the miscarriage rate (O'Leary et al, 2013). In contrast, an American study claimed the approach would be cost-neutral, while decreasing the miscarriage rate by 66% through avoidance of invasive testing following a negative NIPT result and increasing the detection rate of trisomy 21 by 38% due to the superior detection rate of NIPT over FTCS (Garfield and Armstrong, 2014). The ‘Reliable Accurate Prenatal non-Invasive Diagnosis’ (RAPID) group was a 5-year UK national programme aimed at evaluating the possibility of introducing NIPT for aneuploidy screening into standard NHS practice (full publication awaited). It consisted of secondary contingent testing by offering NIPT to any woman who received a FTCS result of between 1: 1 and 1: 1000 (Hill et al, 2014).
Practical considerations of NIPT
Although the benefits of NIPT are unquestionable, there are many other considerations that must be resolved while introducing the test into routine clinical practice:
Current practice at one tertiary English maternity service
St Michael's Hospital in Bristol is the South West of England's tertiary referral centre for fetal medicine and obstetrics, and provides an example of the current practice of aneuploidy screening. An annual audit performed at this hospital showed that, between April 2014 and April 2015, 4561 pregnancies were eligible to receive FTCS and 69.3% (3167 pregnant women) attended for FTCS, with 1628 (35.7%) declining (these proportions are similar to the national average). However, of the 3167 attending, 2632 pregnant women (57.7%) underwent FTCS (the attrition rate was due to wrong dates, unable to obtain an accurate nuchal translucency measurement, or miscarriage). Of the 2632 women undergoing FTCS, 88 women (3.3%) received a high-risk screening result of ≥ 1: 150. Of these 88 women, 42 opted to have an invasive test, and five cases of aneuploidy were diagnosed. The remaining 46 women declined an invasive test and, of these, 39 opted for NIPT (with four cases of aneuploidy being detected). The seven other women declined follow-up. From these limited data, we can conclude that many women are already choosing to opt for NIPT (via the private sector) even before a national guideline has been introduced.
Acceptability of NIPT to pregnant women
Studies investigating the acceptability of NIPT suggest that pregnant women find it preferable because the standard screening is associated with more uncertain results and the invasive procedure with the associated risk of miscarriage (van Schendel et al, 2014). However, NIPT is associated with ethical concerns. Introducing a ‘simple’ blood test to detect Down syndrome may become ‘routinised’ to the point that women may not contemplate the consequences before undertaking the test in the depth they currently do (van Schendel et al, 2015). It is imperative to ensure women are fully informed before making a decision on whether to opt for prenatal testing, through ongoing training of all health professionals caring for pregnant women.
Antenatal Results and Choices (ARC) is a UK charity that provides support to parents through antenatal testing and has detected a marked rise in calls to its national helpline from parents regarding NIPT. Parents contact the charity with a range of questions and concerns as they strive to choose the method of screening and diagnosis that is most acceptable to them. Quotes from parents who have contacted ARC, provided with permission by ARC director Jane Fisher, include:
‘Is it really as good as they say—I mean,
I am 45 and I find it hard to believe that
I am in the clear…’
‘Do I really have to have an
amniocentesis? If the baby has
Down syndrome I'll be too late
to have a surgical termination.’
It is crucial that, if NIPT is introduced as the primary screening offer, health professionals must be fully trained to deliver high-quality counselling to enable informed decision-making by pregnant women.
Summary
The authors are aware that the National Screening Committee is currently consulting on a policy of NIPT implementation. NIPT using cffDNA has already been introduced in the private sector and is rapidly becoming the ‘standard of care’. In our view, its implementation in the NHS will probably be by contingent testing first and then, as the price of the test continues to fall, this will be used as the primary screen. Use of this biological phenomenon is already a major advance in pregnancy care and is bringing advantages to areas other than Down syndrome screening, such as the use of anti-D, recessive disease and investigation of abnormalities seen on ultrasound scanning. Midwives will continue to provide the mechanism by which maternity care is implemented and will need to ensure this advance is delivered in a way that is chosen by pregnant women as a considered ‘opt-in’.