Explaining risk

The way in which the midwife tells a mother about risk will also greatly influence how that risk is perceived. For example, a mother who is told that her risk of a particular condition is 1 in 10 may be more alarmed than if she had been informed that there was a 90% chance of normality. This is known as the ‘framing’ effect (Kessler & Levine 1987).

People vary considerably in the ways that they consider and understand risk, so it is important that this information is presented in a variety of ways. As such, a midwife discussing a 1 in 100 risk of a disorder should also point out the fact that 99% or 99 out of 100 similar people will not experience that disorder. This may help people cope when considering tests or when anxiously awaiting results.

There are other general considerations to take into account when providing information (Hunter 1994):

If a test is undertaken in pregnancy, it is good practice to ensure that parents are clear about how, when and from whom they will be able to obtain the result. If possible, there should be some options available.

Screening tests

Screening tests aim to identify individuals who are most likely to be affected by a named disorder. This makes it possible to target further investigations towards those with the greatest apparent need. Mothers who undergo screening tests will be classified as above or below an action limit for follow-up investigations. Traditionally, this classification has been known as ‘screen positive’ or ‘screen negative’. However, this terminology has caused problems with interpretation. As such, many mothers given a ‘screen positive’ result have assumed that there is a positive certainty that they have an affected pregnancy. Likewise, a ‘screen negative’ result has been interpreted as the exclusion of a problem. In fact, positive or negative in this context simply means that the chances of a problem are higher or lower than specified by an action limit. For this reason, screening results are now referred to as ‘high(er) risk’ or ‘low(er) risk’.

It is also important to note that the action limit (or the dividing point between high and low risk) is usually defined in line with the level of resources available for follow-up procedures. There is no agreed scientific means of calculating what defines high risk. Consequently, some mothers within the low-risk group will be sufficiently anxious to request follow-up, whereas some who are categorized as high risk will not wish to pursue subsequent investigations.

The performance of a screening test is defined in a number of ways:

National fetal screening programmes have been developed for a range of congenital abnormalities. These include fetal anomaly ultrasound screening, Down syndrome screening and haemoglobinopathy screening. These screening programmes should be offered to all pregnant women.

Diagnostic tests

Diagnostic tests are performed in order to confirm or disprove the presence of a particular abnormality. They may be offered as a consequence of screening test results. However, the diagnosis may not give certainty as to the severity of the disorder or the quality of life of a particular individual. Responses to a diagnosis will vary, according to cultural, social, moral and religious beliefs. Furthermore, we are not currently able to diagnose all fetal abnormalities and some will not be manifested until childhood or even adulthood. Examples of diagnostic tests include amniocentesis, chorionic villus sampling (CVS) and ultrasound scans (scans can be used to diagnose some structural anomalies such as spina bifida and they can also be used to screen for clue signs of disorders such as Down syndrome).

Down syndrome screening

Down syndrome (also referred to as Down’s syndrome) is the most common cause of severe learning difficulty in children. In the absence of antenatal screening, around 1 in 700 births would be affected (Kennard et al 1995). While some children with Down syndrome learn literacy skills and lead semi-independent lives, others remain completely dependent. Around one in three of these babies are born with a serious heart defect. The average life expectancy is about 60 years, although most people develop pathological changes in the brain (associated with Alzheimer’s disease) after the age of 40 (Kingston 1994).

Maternal blood contains a number of hormones that can be used to assess the risk of Down syndrome. A growing number of hormones have been shown to have different average levels in Down syndrome compared with unaffected pregnancies and are therefore known as biochemical markers. These are shown in Table 18.1. One of the difficulties with using biochemical markers for screening is the fact that their levels fluctuate and are dependant on a number of factors, such as maternal weight. Consequently, a more accurate risk estimate can be obtained if several markers are used in combination. Levels of biochemical markers are never diagnostic for Down syndrome. The incidence of Down syndrome is also related to maternal age, so this is also factored into risk calculations. Older mothers are more likely to receive a higher risk result. The risk of Down syndrome by maternal age is shown in Table 18.2.

Table 18.1Biochemical markers used in Down syndrome screening

Table 18.2Estimated risk of having a Down syndrome birth according to maternal age at birth (calculated using eight surveys)

Biochemical markers, combined with maternal age, form part of the national Down syndrome screening programme. The UK National Screening Committee (2004) standards state that, whatever markers are used, tests should have a detection rate of >75% with a false positive rate of <3%. The cut-off point between higher and lower risk is 1:250. Some centres also add ultrasound markers (see Nuchal translucency measurement, p. 299) to maternal age and biochemical screening tests. This greatly enhances the reliability and safety of Down syndrome screening. The aim is to maximize the detection rate, while minimizing the false positive results. This is very important because women with higher-risk results can only find out whether the baby is affected by undergoing invasive procedures such as amniocentesis or CVS. These tests carry a risk of causing a procedure-induced miscarriage. The types and safety levels of different screening tests are shown in Table 18.3, which shows increasing safety and effectiveness as more markers are added together.

Table 18.3Types and safety of Down syndrome screening tests

It is noteworthy that the serum integrated and integrated tests are performed in two stages, with first and second trimester testing. Some clinicians find this concept problematic because it necessitates withholding test results until all phases of the testing process have been completed. Research is currently underway to explore the feasibility of this model in mainstream practice. One possible modification is known as contingency screening. A two-phase testing process occurs, but women above or below certain thresholds receive their results after the first phase. Higher risk mothers are offered diagnostic testing at this point and lower risk mothers are informed that their risk is low. Women in the middle range proceed to second trimester testing, since further information is advantageous in determining the risk classification.

When performing serum screening, it is essential that the gestation is accurately assessed. The scan dates should be used in preference to menstrual history, since scans give a more accurate indicator of the time of conception. Crown–rump length or biparietal diameter scan measurements should be used. Femur length is not a reliable indicator of gestational age. Serum screening is not indicated in multiple pregnancies because it is impossible to know how the levels relate to each fetus. Mothers with multiple pregnancies should be offered nuchal translucency scanning as an alternative. Serum levels also change with maternal weight (i.e. they are more concentrated in women with lower body mass and circulating volume). It is therefore important to weigh the mother on the day of the test.

Neural tube defect screening and AFP

Traditionally, α-fetoprotein (AFP) has been used as a biochemical marker for neural tube defects such as spina bifida. This test has been superseded by the use of ultrasound to diagnose a range of neural tube defects. However, many centres still report high AFP levels at 15–20 weeks because this can be a prognostic indicator for other problems. Around 2% of mothers have a raised AFP level (Kennard et al 1995). Reasons include multiple pregnancy, incorrect gestation and threatened miscarriage. Raised AFP levels can be predictive of intrauterine growth restriction and pre-eclampsia. These pregnancies are therefore considered by some to be high risk and some centres offer increased fetal growth and fetal well-being monitoring in the third trimester.

Haemoglobinopathy screening

The NHS antenatal and newborn screening programmes include antenatal screening for fetal haemoglobinopathies. This should be linked with the newborn bloodspot screening programme, which tests for sickle cell disease. Haemoglobinopathies are inherited disorders of haemoglobin and are more prevalent in certain racial groups (NHS Sickle Cell and Thalassaemia Programme 2004; see Ch. 21).

Currently, antenatal screening is organized differently in different areas, based on population prevalence. High prevalence areas offer all pregnant women electrophoresis screening for haemoglobin variants and thalassaemia trait. Low prevalence areas use a family origins questionnaire to determine genetic ancestry for the last two generations (or more if possible). Women with genetic ancestry that includes high-risk racial groups are then offered electrophoresis testing. If the mother is found to be a haemoglobinopathy carrier, partner testing should be offered. Genetic ancestry is also important when interpreting screening results. It is also important to establish maternal iron levels when carrier status is suspected, since iron deficiency can give rise to similar red cell appearances. Haemoglobinopathies are recessively inherited, so the fetus would have a 1 in 4 chance of inheriting the disorder and a 1 in 2 chance of being a carrier. Diagnostic CVS or amniocentesis should be offered when both parents are carriers of a haemoglobinopathy.

Safety aspects of ultrasound

Ultrasound has been used as a diagnostic imaging tool since the 1950s, so we are now into the third generation of scanned babies. It seems reasonable to assume that any major adverse effects of this technology would have become apparent before now. However, modern machines have higher resolutions and indications for ultrasound scanning have greatly increased. This means that levels of exposure to ultrasound have increased in pregnancy and there is little research into the effects. Ultrasound should be used with respect and only when there is good indication.

Ultrasound waves have been shown to cause tissue heating, primarily within the first 40 s of exposure (Bosward et al 1993). There are also conflicting data regarding a possible association with low birth weight (Newnham et al 1993, Salvesen 1997), with dyslexia (Stark et al 1984) and with non-right-handedness (Salvesen et al 1993). Consequently, care should be taken to limit exposure time and the thermal indices should be controlled (European Committee of Medical Ultrasound Safety 2006). The use of ultrasound for entertainment or as a ‘keep sake’ is not recommended. Ultrasound is a diagnostic tool, but diagnosis can only be as reliable as the expertise of the operator and the quality of the machine. As Wood (2000) states, abnormalities may be missed or incorrectly diagnosed if the operator is inexperienced or inadequately trained.

Women’s experiences of ultrasound

In general, women experience ultrasound as a pleasurable opportunity to have visual access to their unborn baby (Sandelowski 1994). Indeed, ultrasound scans have been shown to increase psychological attachment to the fetus (Sedgman et al 2006). Parents have a profound curiosity about their baby and a scan can turn something nebulous into something which seems much more real as a living individual (Furness 1990). This can be particularly important for a woman’s partner and family, who do not have the immediate physical experience of the pregnancy. Women tend to regard their scan as providing a general view of fetal well-being: the fact that the fetus is alive, growing and developing. However, this reassurance is temporary and begins to wear off after a few weeks (Clement et al 1998). Mothers may then seek other forms of reassurance (e.g. monitoring fetal movements, auscultation of the fetal heartbeat). This initial reassurance may also create an enthusiasm for scans when there is no clinical indication.

However, scans may also cause considerable anxiety, particularly if there is a suspected or actual problem with the fetus. There is evidence to suggest that women who miscarry after visualization of the fetus on scan may feel a heightened sense of anguish because the fetus seemed more real. This may also be the case for parents considering termination of pregnancy on the grounds of fetal abnormality. However, others may view their scan as a treasured memory of the baby they lost (Black 1992).

The identification of fetal abnormality in the antenatal period has differing psychological effects for parents when the pregnancy is to continue. Some parents have reported feeling grateful that they were able to prepare for the birth of a child with a disability (Chitty et al 1996). However, others have reported feelings of wishing they had not known about their child’s problems before birth because this created a powerful image of the fetus as a ‘monster’. Some parents reported this to be far worse than the reality of caring for the baby after birth (Turner 1994). It is necessary for midwives to be mindful of the powerful psychological effects ultrasound scans have on pregnant women and their families, if sensitive and appropriate care is to be given at this potentially distressing time.

The midwife’s role concerning ultrasound scans

As for all procedures, mothers should be fully informed about the purpose of the scan. Information should be given about which conditions are being checked for and which problems the scan would be unable to detect. Because of the pleasurable aspect of seeing the fetus, ultrasound scans have traditionally been tests that mothers undertake willingly, without prior discussion and consideration of potential consequences. It is advisable to remind mothers gently of the medical indications for scans, so that they can decide whether or not they wish to undergo a procedure that may bring unwelcome news. Women should be aware that ultrasound scans are optional and not an inevitable part of their care.

There is evidence that, although some mothers may find this information disturbing, most feel that this is outweighed by the positive aspects of seeing the baby and gaining reassurance (Oliver et al 1996). Indeed, extra information about the purpose of the scan has been shown to increase women’s understanding and satisfaction with the amount of information received, while the proportion of women accepting a scan (99%) appears to remain unchanged (Thornton et al 1995).

The Royal College of Obstetricians and Gynaecologists recommends that, wherever scans are performed, a midwife or counsellor with a particular interest or expertise in the area should be available to discuss difficult news. Discussion about the implications of this should also take place with an obstetrician within one working day (RCOG 2000). Effective multidisciplinary team working and communication are therefore essential. It is also good practice for the midwife to liaise with the primary healthcare team, who would normally carry out the majority of antenatal care. With the increasing use of client-held records, mothers may have more opportunity to scrutinize the written results of their scan. Midwives may increasingly be called upon to explain and discuss these findings, both in hospital and in the community setting.

First trimester pregnancy scans

All women should be offered a first trimester scan. The purpose of this is to establish:

There is evidence to suggest that at least one scan is beneficial, mainly in reducing the need to induce labour for post-maturity (Neilson 1999). A gestation sac can usually be visualized from 5 weeks’ gestation and a small embryo from 6 weeks. Until 13 weeks, gestational age can be accurately assessed by crown–rump length (CRL) measurement (the length of the fetus from the top of the head to the end of the sacrum). This is demonstrated in Figure 18.2. Care must be taken to ensure that the fetus is not flexed at the time of measurement. Mothers are asked to attend with a full bladder, since this aids visualization of the uterus at an early gestation.

Figure 18.2 Crown–rump length.

Measurement of nuchal translucency at 10–14 weeks

Additional information about the fetus can be gained by measuring the nuchal translucency (NT) at 10–14 weeks’ gestation. This is the thickness of the subcutaneous collection of fluid at the back of the neck, as shown in Figure 18.3. During the last decade, a series of studies have reported that increased NT is associated with chromosomal abnormalities, as well as other structural and genetic disorders (Nicolaides et al 1999). This information can be used in combination with maternal age and biochemical markers for Down syndrome.

Figure 18.3 Nuchal translucency measurement.

The main advantage of this test is that it offers an early way of assessing the mother’s risk for Down syndrome. In general, mothers greatly value the opportunity for early information about Down syndrome, so that they could consider the option of termination before they are visibly pregnant and can feel their baby’s movements. Increased NT is also associated with other structural (mainly cardiac) and genetic syndromes, so increased pregnancy surveillance could be arranged. However, a disadvantage of this knowledge is that parents may suffer considerable anxiety until later scans offer some degree of reassurance.

Another potential disadvantage is that early identification of chromosomally abnormal pregnancies may mean that parents are faced with a decision about whether to terminate a pregnancy that may be destined to miscarry naturally. Approximately 40% of affected fetuses die between 12 weeks’ gestation and term (Nicolaides et al 1999). Some parents may experience more feelings of guilt after a termination than they would have done had the pregnancy spontaneously miscarried.

Second trimester ultrasound scans

After 13 weeks of pregnancy, gestational age is primarily assessed using the biparietal diameter (BPD). This is the measurement between the two parietal eminences of the fetal skull. It is a very useful measurement during the second trimester, but becomes less accurate towards the end of pregnancy because the shape of the head may alter. Limbs are also measured, most notably the femur.

The detailed fetal anomaly screening scan

This scan is usually performed at 18–20 weeks of pregnancy, since visualization of fetal anatomy is more difficult before that time (Drife & Donnai 1991). The purpose of this scan is to reassure the mother that the fetus has no obvious structural anomalies that fall into the following categories:

• anomalies that are incompatible with life

• anomalies that are associated with significant morbidity and long-term disability

• anomalies which may benefit from intrauterine therapy

• anomalies which may require postnatal treatment or investigation.

Detection rates vary considerably, but it is thought that around 50% of significant abnormalities are identified at this time (Boyd et al 1998). This is influenced by the expertise of the sonographer and quality of the equipment. There may also be technical difficulties, such as fetal position, multiple pregnancy or maternal obesity.

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Also, some structural problems do not have associated sonographic signs. An example of this would be tracheo-oesophageal fistula (an opening between the trachea and the lower oesophagus). Moreover, some fetal abnormalities, such as hydrocephalus and bowel obstructions, may not appear until later in pregnancy. Diagnosis may therefore be missed. Average detection rates for some abnormalities are presented in Table 18.4. It is vital that mothers are fully aware of the precise purpose and limitations of the detailed scan. The comprehensive range of structures to be examined during the detailed scan are presented in Box 18.2. Examination of the four chambers of the fetal heart is shown in Fig. 18.4.

Table 18.4 Detection rates of fetal abnormalities, if present, on detailed ultrasound scan

Problem	Chance of being seen (%)
Spina bifida	90
Anencephaly	99
Hydrocephalus	60
Major congenital heart problems	25
Diaphragmatic hernia	60
Exomphalos/gastroschisis	90
Major kidney problems	85
Major limb abnormalities	90
Cerebral palsy	Never seen
Autism	Never seen
Down syndrome	May be associated with heart or bowel problems in about 40%

RCOG (2000)

Box 18.2 Features examined on detailed fetal ultrasound scans

From RCOG 2000

• Spine

• Head shape and internal structures (cavum pellucidum, cerebellum, ventricular size at atrium <10 mm)

• Abdominal shape and content – at level of the stomach

• Abdominal shape and content – at the level of kidneys and umbilicus

• Renal pelvis <5 mm

• Longitudinal axis, abdominal–thoracic appearance (diaphragm/bladder)

• Thorax – at level of four chamber cardiac view

• Arms – three bones and hand

• Legs – three bones and foot

• Face and lips

• Cardiac outflow tracts.

Figure 18.4 Four chamber view of the fetal heart. The opening between the atria (foramen ovale) can be seen.

Markers for chromosomal abnormality

Markers are minor sonographic clue signs, which may increase the chance that the fetus has a chromosomal abnormality (most are associated with Down syndrome). They are seen in many normal fetuses (at least 5%) and, when isolated, are of dubious value (Whittle 1997). Examples of such markers include the following:

Choroid plexus cyst This is a collection of cerebrospinal fluid within the choroids plexi, from where cerebrospinal fluid is derived.

Echogenic bowel This is a bright appearance of the bowel, equivalent to the brightness of bone. It is also associated with intra-amniotic bleeding and fetal swallowing of bloodstained liquor (Fig. 18.5).

Nuchal fold >5 mm at 20 weeks’ gestation This is an increased thickness of fetal skin and fat at the back of the fetal neck. Subcutaneous fluid (NT) cannot usually be visualized after 14 weeks.

Echogenic foci in the heart These are bright echoes from calcium deposits in the fetal heart, often the left ventricle. They do not affect cardiac function.

Dilated renal pelvis >5 mm This may be due to slight backflow from the ureters and is more common in male fetuses.

Short femur This is a shorter than average thigh bone, when compared with other fetal measurements.

Sandal gap This is an exaggerated gap between the first and second toes.

Figure 18.5 Echogenic bowel.

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The strength of association between each individual marker and Down syndrome varies considerably. As such, an increased nuchal fold increases the chances of an affected pregnancy by 10 times the background risk. Conversely, echogenic foci in the heart increase the chances only by a marginal factor of 1.2 (Snijders & Nicolaides 1996). The UK National Screening Committee recommends that the Down syndrome screening risk should not be altered on the basis of a single ultrasound marker, unless it is at least as significant as an increased nuchal fold. It is also noteworthy that there is much debate about the validity of some of the above markers, particularly echogenic foci in the heart. It is possible that sonographers will not continue to report some of these markers in the future. Likewise, many new markers are emerging and may be incorporated into national screening programmes in the future. One example is the absence of nasal bone.

Advantages and disadvantages of fetal anomaly scans

Providing the sonographer has sufficient expertise, many lethal or severely disabling conditions can be detected during the 18–20 week scan. There is also an increase in first trimester diagnosis. Although this means that parents may be faced with difficult and unexpected decisions, it may be that later psychological trauma and physical suffering can be prevented. Furthermore, many parents are offered reassurance that no obvious abnormalities were seen. There is also evidence that, for neonates requiring early surgical or paediatric interventions, prior knowledge of the abnormality allows a plan of care to be evolved in advance of the birth. The mother can then give birth in a unit with appropriate facilities. This has been shown to reduce morbidity in the cases of gastroschisis (an abdominal wall defect, adjacent to the umbilicus, allowing the intestines and other abdominal organs to protrude outside the body), cardiac abnormalities and intestinal obstruction (Chang et al 1991, Romero et al 1989).

However, there is also the potential for false positive findings that are not confirmed postnatally. In particular, markers for chromosomal abnormalities may cause considerable anxiety without having any clinical significance for the baby. Indeed, they may be regarded as variants of normal, particularly if the fetal chromosomes are proven to be normal (see ‘A testing time’, Box 18.3).

Figure 18.9 The scan of the twins.

Figure 18.10 The twins.

There is also the problem of defining the prognosis for some recognizable abnormalities. In summary, the 18–20 week scan appears to confer psychological and health improvement benefits in some cases, but also has the capacity to cause great anxiety and distress. Care must be taken to ensure that parents are fully informed of the purpose, benefits and limitations of ultrasound scans before they consent to this procedure.

Third trimester pregnancy scans

In general, late pregnancy scans are performed in response to a specific clinical need and not as a screen of the low risk pregnant population. However, fetal abnormalities may come to light or be reassessed at this time. Many late scans are performed as a means of monitoring fetal well-being, growth and development.

Doppler ultrasonography

Placental blood flow can be assessed using the Doppler shift (a change in the frequency of ultrasound wave reflection, according to the speed and direction of blood flow). Compromises in maternofetal circulation can be identified. Abnormalities in Doppler measurements may be detected before growth becomes impaired and can be used as a prognostic indicator. Doppler analysis of the umbilical artery is the only test that improves outcomes in high-risk pregnancies (Neilson & Alfirevic 2005). This is shown in Figure 18.6.

Figure 18.6 Measurement of umbilical artery Doppler flows.

Findings from growth scans, biophysical profile scores and CTG recordings should be considered collectively, taking into account the full clinical picture and obstetric history.

Chorionic villus sampling

Chorionic villus sampling is the acquisition of chorionic villi (placental tissue) under continuous ultrasound guidance. Chorionic villi originate from the same cells as the fetus and therefore generally have the same genes and chromosomes. This may be performed at any stage after 10 weeks of pregnancy.

Access may be achieved transcervically (until 13 weeks’ gestation) or via the transabdominal route. When CVS is performed transcervically, a catheter is introduced through the cervix and is guided into the chorion frondosum. Suction is then applied to an attached syringe and 10–40 mg of tissue are aspirated. This procedure is represented in Figure 18.8.

Figure 18.8 CVS procedures. (A) transcervical aspiration; (B) transabdominal puncture.

The transabdominal approach involves the insertion of a needle into the maternal abdomen. Under ultrasound guidance, the needle is pushed through the uterine wall and into placental tissue. Tissue is aspirated via an attached syringe. Villi are carefully separated from maternal decidua under a dissection microscope, prior to cytogenetic (chromosome) analysis.

In general, there are two stages to the reporting of results. The initial result is issued after 24–48 hrs and counts specific chromosomes (e.g. to test for Down syndrome). Cells are then cultured for 14–21 days, to confirm the initial results and to allow more detailed cytogenetic examination of the chromosome structure. DNA (for genetic) or biochemical (for metabolic) diagnoses can also be obtained from chorionic villi tissue.

The main advantage of CVS is that this is the earliest way mothers can obtain definitive information about the chromosomal/genetic status of the fetus. Mothers who are known carriers of particular disorders often have high recurrence risks and so are anxious until their results are known. If the news is not good and mothers wish to end their pregnancy, a surgical procedure can be performed. After 12–13 weeks, terminations are usually induced medically, in order to reduce the risk of causing cervical incompetence. This process of induction and vaginal birth can be very distressing. Decisions about termination of pregnancy may also become more difficult as the pregnancy progresses, since the mother begins to feel fetal movements and becomes visibly pregnant. Most women considering diagnostic procedures prefer to have an early test (Abramsky & Rodeck 1991). The main disadvantage with CVS is the procedure-induced risk of miscarriage. This is 0.5–2%, depending upon the experience of the operator (National Electronic Library for Health 2001) and occurs because of infection or bleeding. Also, ambiguous results are obtained in 1% of samplings (Holzgreve et al 1999).

Early CVS has been associated with limb reduction abnormalities. However, a large WHO international trial found the incidence of these abnormalities to be equal to the background rate (6/10 000), providing CVS was performed after 10 weeks. If CVS is performed prior to this gestation (i.e. during organogenesis), there is a higher chance that it may be harmful. Consequently, the WHO recommends 10 weeks as the earliest safe gestation for the procedure (Froster & Jackson 1996). In general, if an experienced operator with sufficient counselling support performs CVS, it is an effective and safe means of gaining early information about the fetus.

Amniocentesis

This is usually performed after 15 weeks’ gestation, since early amniocentesis has a higher loss rate than early CVS (Nicolaides et al 1994). The procedure involves transabdominal insertion of a fine needle into the amniotic fluid cavity, under continuous ultrasound guidance; 15 mL of amniotic fluid are aspirated. Cytogenetic, molecular (DNA) and biochemical analyses are possible. Amniocytes are often examined. These comprise cells that have been shed from several fetal sites, including skin, lungs and renal tract. The risk of procedure-induced miscarriage is 1% (National Electronic Library for Health 2001), although 2–3% of mothers have some leakage of amniotic fluid (Simpson et al 1981). Miscarriage is usually caused by infection or spontaneous rupture of the membranes at the puncture site.

Amniocentesis has traditionally been performed more commonly than CVS, mainly because more obstetricians had the required training. However, recent clinical governance initiatives have resulted in a move towards performing invasive procedures only in specialist centres. This is important because procedure loss rates are so dependent on the operator. In the second trimester of pregnancy, CVS and amniocentesis have similar risks and benefits. The miscarriage risks are comparable and, in most cases, both tests provide the required information.

Recent advances in cytogenetic techniques mean that mothers can obtain an initial set of results (usually for Down syndrome) and then a full culture result after 2–3 weeks.

Fetal blood sampling

The use of this technique has declined in recent years because improved molecular and cytogenetic techniques allow more diagnoses to be made from chorionic villi or amniotic fluid. However, fetal blood may be advantageous when there are ambiguous findings from placental tissue. Also, when there is Rhesus isoimmunization, it may be necessary to determine the fetal haemoglobin. When this is low, an intrauterine transfusion may be performed. Blood can be sampled from the umbilical cord or intrahepatic umbilical vein. The latter is less risky, as there are no umbilical arteries in close proximity. The loss rate also depends upon the gestation and condition of the fetus. In uncomplicated procedures after 20 weeks, the loss rate is around 1% (Holzgreve et al 1999).

Rapid diagnostic techniques

Rapid diagnostic technologies are now available to women who undergo invasive testing. There are two main techniques in use today, FISH and QF-PCR.

Fetal imaging techniques

Ultrasound scans in pregnancy have been discussed at length in this chapter, since they are important fetal investigations. Women generally see two-dimensional (2D) images of their unborn baby. However, there is a growing market for three-dimensional ultrasound imaging (3D). As such, multiple images are stored digitally and then shaded to produce life-like pictures. This technique can assist the diagnosis of surface structural anomalies, such as cleft lip and spina bifida. However, there are a growing number of commercial enterprises that offer 3D or 4D (3D in real time) ‘bonding’ scans. This is a matter of concern, since such services have minimal regulation and vary considerably in their quality and marketing techniques. The level of pre- and post-test advice is also questionable in some cases.

Magnetic resonance imaging (MRI) has also been applied in the examination of the fetus over the last two decades. This technique has not been widely applied because ultrasound can give similar diagnostic information at a lower cost. However, MRI may have a contribution to make, particularly when examining the brain. There is evidence that this may provide additional information and change the counselling and management for a significant number of pregnancies where brain abnormalities are suspected (Levine et al 1997). A further possible application is that MRI could offer an alternative to postmortem following termination or perinatal death. This may offer information to parents who decline postmortem because of its invasive nature (Brookes & Hall-Craggs 1997). MRI may also be useful in providing serial brain images following asphyxia in the newborn. This may give us a better understanding of the evolution of brain injury (Maalouf et al 1998). More recently, MRI imaging has been used to refine the diagnosis of diaphragmatic hernias and sacrococcygeal teratomas (Kumar & O’Brien 2004).

Fetal cells in the maternal circulation

There has been extensive research into the use of fetal cells in maternal blood for genetic diagnosis. This would be helpful to mothers who want diagnostic information without the risk of procedure-induced miscarriage of a potentially normal fetus. Some progress has been made, such as fetal sexing and rhesus typing from maternal blood. However, this technology is not yet sufficiently well developed for wider application to general clinical care.

Fetal therapy

There are certain instances when therapeutic interventions may improve fetal prognoses. For instance, therapeutic amniocentesis may be performed to drain excess liquor. This may reduce the likelihood of pre-term labour when the uterus is large-for-dates. Therapeutic amniocentesis is also sometimes performed in monochorionic twin pregnancies with twin-to-twin transfusion syndrome (discordant placental circulation and consequent discordant growth and liquor volume – if severe, this has a poor prognosis for both babies). Liquor is drained from the largest sac; this sometimes helps equal the pressures between the twins and allows both fetuses to grow satisfactorily.

Another treatment that is sometimes used is laser treatment to reduce pathological placental flow between the twins. Shunts can also be inserted into the fetus in order to drain pathological collections of fluid. These include ascites, renal obstruction and hydrocephalus. However, while there may be some improvement in outcome, much will depend on the underlying cause of the obstruction.