Chapter 8. Antenatal screening for fetal abnormality
Introduction
Antenatal screening for fetal abnormality is an integral part of routine antenatal care. It is often assumed that women will want to take up this service, just as they would have their urine tested or blood pressure checked. However, what has become routine practice for midwives can have far reaching implications for women. This aspect of antenatal care requires the midwife to be particularly receptive to the cues that women give her and to respect the choices they make. This chapter focuses on antenatal screening for Down’s syndrome to illustrate some of the principles that are relevant to screening generally. Tests used for the diagnosis of congenital conditions are summarized, as women need to know what a positive screening test result might lead to.
What is screening?
The terms ‘screening’ and ‘testing’ are often used synonymously. However, they are distinctly different and it is important that the midwife is able to convey this difference to women.
Think about anyone you know with a child who has either a physical or mental disability.
Consider how caring for a disabled child impacts on family life.
Think about the decisions you would make about screening for fetal abnormality if you were expecting a baby.
Screening is a health service in which members of a defined population, who do not necessarily perceive they are at risk of a disease or its complications, are asked a question or offered a test, to identify those individuals who are more likely to be helped than harmed by further tests or treatment.
A population is screened to identify people who would benefit from further investigation; that is, those who have a higher risk of the condition being screened for. However, when resources are scarce or when the test itself may potentially cause harm, screening is often limited to a sub-section of a population already deemed to be at higher risk than the rest. As Green & Statham (1993:124) succinctly state, ‘if we had cheap, accurate, risk-free diagnostic tests, we would apply them to everybody.’
When screening identifies an individual who has a ‘high risk’, diagnostic testing is offered to confirm or exclude the condition. Screening usually precedes diagnostic testing; however, individuals already known to be ‘high risk’ may opt straight for testing if it is offered. The aim of diagnostic testing is to identify if the condition is present and, in the case of antenatal diagnosis for Down’s syndrome, to offer the woman the options of continuing with her pregnancy or having a termination.
False positive and negative
Screening is not diagnostic, and it is important that the midwife understands what is meant by a positive or negative screening result. Some results will be positive, that is, fall into an at-risk group, the range of which has previously been determined. However, not all of the cases that fall into that group will have the condition, and they are termed ‘false positives’; a result that indicates there is a problem when there is not one.
Some results will be negative; that is, they fall outside of the previously determined at-risk range. However, not all of these cases will be free of the condition being screened for, and these are termed ‘false negatives’; a result that indicates there is not a problem, when there is.
These are important considerations, particularly where positive screening may lead to invasive testing in order to confirm the presence or absence of the condition, as is the case with antenatal screening for Down’s syndrome. An ideal screening test would be sufficiently sensitive to detect a high proportion of those at risk, without subjecting a large number of people to unnecessary diagnostic testing. The national programme for Down’s syndrome screening has a target to detect more than 75% of cases and for a false positive rate of less than 3% (National Screening Committee (NSC) 2007a).
The screening test should meet the following criteria stated by the NSC (2003):
1. There should be a simple, safe, precise and validated screening test
2. The distribution of test values in the target population should be known and a suitable cut-off level defined and agreed
3. The test should be acceptable to the population
4. There should be an agreed policy on the further diagnostic investigation of individuals with a positive test result and the choices available to those people.
Following wide consultation, standards have been developed and agreed by the NSC (2007c), that reflect both generic issues related to antenatal screening and those specific to Down’s syndrome. Those relating to antenatal screening tests include policy arrangements, clinical arrangements, education and training for staff: information and support for women and their partners and audit and monitoring processes. Those specific to Down’s syndrome comprise: audit and monitoring for Down’s syndrome, laboratory standards for Down’s syndrome serum screening and ultrasound standards for Down’s syndrome screening.
Screening for Down’s syndrome
The incidence of Down’s syndrome is 1 in 800 births in the general population. This increases with advancing maternal age, from 1 in 1500 births at age 20, to 1 in 280 births at 36 years and 1 in 30 at age 44 (Fetal Anomaly Screening Programme 2007). As the age at which women have their babies increases, finding accurate and acceptable means of screening for Down’s syndrome continues to be a relevant issue.
Down’s syndrome could potentially occur in any pregnancy. However, women over 35 years of age are particularly at risk of carrying a baby with Down’s syndrome and thus maternal age has been a long-standing screening test for this condition. Such ‘at-risk’ women are often given the option of diagnostic testing, although maternal age as a screening test has limited sensitivity, as approximately 70% of cases of Down’s syndrome occur with women under the age of 36 years (Mutton et al 1998). Because of this, and the introduction of new screening tests, screening for Down’s syndrome is now offered to all women in early pregnancy (NICE 2008). Screening tests used to identify women at risk of carrying a baby with Down’s syndrome include serum screening and ultrasound.
Current provision of screening for Down’s syndrome is variable across the UK. A retrospective study, exploring the outcome of 155501 births over the period 1994–1999, highlighted the variation in screening policies between eight district general hospitals in one health region (Wellesley et al 2002). Screening methods included some or all of the following: maternal age, serum screening, ultrasound scans for nuchal thickness, anomaly and gestational age. Only two hospitals had the same screening policy, and no significant advantage was found for any particular policy, although detection rates ranged form 48% to 58%. Following a survey of consultant obstetricians’ attitudes towards screening for Down’s syndrome, Green (1994) reported that some obstetricians mentioned the need for a national policy. Such national guidance is now available (NICE 2008) and aims to minimize conflicting information and disparity, through the promotion of clinical practice based on the available evidence. Each Trust should employ a Screening Coordinator to facilitate the implementation of evidence-based policy and guidance (NSC 2007a).
What is serum screening?
Serum screening for Down’s syndrome uses a maternal venous blood sample taken by the midwife or phlebotomist. The woman must have had detailed information on which to base her decision to consent to the test. There is a range of serum tests available (Table 8.1). However, following an extensive review of the literature, the National Institute for Health and Clinical Excellence recommends that women who request antenatal screening for Down’s syndrome are offered the ‘combined test’ between 11 weeks 0 days and 13 weeks 6 days (NICE 2008). If the pregnancy is further advanced, or the nuchal translucency cannot be measured, women should be offered either the ‘triple’ or ‘quadruple’ serum screening test, between 15 weeks 0 days and 20 weeks 0 days.
| Pregnancy-associated plasma protein A (PAPP-A) Beta-human chorionic gonadotrophin (hCG) Unconjugated oestriol (uE3) Alpha feto protein (AFP) | ||
| When | Name | Markers |
|---|---|---|
| 11 weeks–13 weeks 6 days | Combined test | Serum hCG Serum PAPP-A Ultrasound nuchal translucency |
| 15 weeks–20 weeks | Double | Serum hCG Serum uE3 |
| 15 weeks–20 weeks | Triple | Serum hCG Serum uE3 Serum AFP |
| 15 weeks–20 weeks | Quadruple | Serum hCG Serum uE3 Serum AFP |
Measurement of markers (Table 8.1) detected in maternal serum is combined with maternal and gestational age, in the calculation of individual risk. Adjustments are made according to maternal weight, so this must be recorded when the test is taken. It has been established that other maternal characteristics, such as ethnic origin, conception via in vitro fertilization and smoking can influence the levels of serum markers in the blood (Kagan et al 2008). Agreed standards for antenatal screening state that ‘the cut-off level used to define the population at increased risk of a Down’s syndrome affected pregnancy must be 1 in 250 at term’ (Fetal Anomaly Screening Programme 2007).
Ultrasound screening for Down’s syndrome
The use of scanning in the UK is accepted practice. Not only do women expect to have at least one scan during their pregnancy, but they also expect that they will be able to purchase a picture of their unborn baby and in some cases be informed of the baby’s sex. Seeing the baby on screen helps make the pregnancy seem more real, for both the woman and her partner (Santalaahti & Hemminiki 1998).
The sensitivity of the scan performed will depend on the resolution of the equipment used, the skill of the ultrasonographer, the gestation and the position of the baby.
From a clinical point of view, the use of routine scanning to date the pregnancy has been justified over the use of last menstrual period (LMP) because it has been shown to reduce the number of inductions for post-maturity (Hogberg & Larsson 1997). Estimation of gestational age by ultrasound rather than on menstrual history also improves the accuracy of serum screening tests (Brennand & Cameron 2001). In 2002 the Royal College of Obstetricians and Gynaecologists (RCOG 2002) recommended that all women be offered at least one scan to confirm gestational age, preferably followed by an anomaly scan at around 20 weeks’ gestation. National guidance recommends that women have a dating scan after 10 weeks and a fetal anomaly scan between 18 and 20 weeks plus 6 days.
A systematic review of women’s views of ultrasound scanning during pregnancy indicates that women often lack information regarding the purpose of the scan (Garcia et al 2002). Women were often ignorant that one aim of scanning was to look for markers associated with Down’s syndrome. Others assumed that the scan was compulsory and some had unrealistic expectations that the scan confirmed normality.
Nuchal translucency thickness
The fluid that collects behind the neck of the fetus is increased in those who have an abnormality. It can be detected by ultrasound scan and measured; only 10% with a measurement of 6mm or more will be normal (Berger 1999).
Snijders et al (1998) reported an 82% detection rate for Down’s syndrome through the use of a combination of maternal age and fetal nuchal translucency thickness, measured by ultrasonography between 10 and 14 weeks, to select women who were at risk and hence offered diagnostic testing.
Information for women
The booking history is often the first time that women have the opportunity to discuss the issue of screening for fetal abnormality. However, the information that women receive is sometimes less than informative. Women may access information that is brief and misleading. An Australian study revealed that information stated that screening was to make sure that the pregnancy was progressing well, but gave no information about the limitations or reliability of screening (Searle 1997). The increase in the number of options available also adds to the confusion for prospective parents (Saller & Canick 2008).
A content analysis of 80 leaflets available to women in the UK about serum screening for Down’s syndrome (Bryant et al 2001) revealed that one third did not contain any descriptive information about the condition. They concluded that more attention needed to be paid to the tone, content and the needs of the reader. A study of 34 women in South Wales found that half the pregnant women had no knowledge of Down’s syndrome (Al-Jader et al 2000). The National Screening Committee now produces a comprehensive booklet that provides detailed information about the conditions that can be screened for and the implications of the tests (NSC 2007a).
Lack of knowledge is not confined to pregnant women. In a survey involving 162 midwives in a large Trust, 41% demonstrated poor knowledge related to Down’s syndrome screening and only 39% of the midwives who responded felt confident to counsel women adequately about screening (Samwill 2002). Perceived ability to facilitate informed choice in antenatal screening was also limited in a study conducted by Wray & Maresh (2000) and again echoed in a recent Australian study (Noseworthy & Cooper 2007). Education and training of midwives regarding all aspects of screening is the responsibility of local screening coordinators, in liaison with pre- and post-registration education providers and local Trusts. However, individual midwives must also take responsibility to keep up-to-date with new practices and technologies (NMC 2008). There is a plethora of resources to support midwives in this challenging aspect of their role (Harcombe 2007).
The midwife must be aware that the choices women and their partners make about antenatal screening for fetal abnormality are not solely based on the information they receive in pregnancy. They may have had long-standing views following the experiences of their family and friends. Religious and cultural values will also impact on the course that women take. Knowing that they would never terminate a pregnancy, for whatever reason, will prevent some women from having a screening test. Other women may be against termination but feel that they want to know if their baby has an increased risk of having the condition. Some women will feel strongly that they would want their baby, whatever problems it had. Others might feel that it is wrong to birth a baby that might have a reduced quality of life. That individuals come to a parent partnership with potentially different views also means that they need to find a stance from which they both feel comfortable, and this may take time.
Midwives must also be aware of the impact that their body language has on their interactions with women, and demonstrate to each woman that they are listening (Baston 2002). Stapelton et al (2002)
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
