Identifying placenta praevia, placenta accreta, vasa praevia and uterine scar

Ultrasound, and specifically transvaginal assessment of the cervix, is also useful for third‐trimester assessment of placental position and vasa praevia. Up to 20% of placentas may be identified as being low‐lying during a transabdominal 18–20 week morphology scan [15]. This cohort are recognized as being at risk of having placenta praevia, with implications for risk of antepartum haemorrhage and mode of delivery. Women who have a low placenta at 20 weeks are typically offered another scan at 34–36 weeks’ gestation to confirm placental position [16]. The timing of this assessment is a compromise between allowance of adequate time for development of the lower uterine segment and of performance of the scan before the onset of spontaneous labour. The leading edge of the placenta is often best assessed using a transvaginal approach, particularly if the placenta is posterior, as the leading edge may be masked by the sonographic signal returned from the presenting part, particularly if the fetus is cephalic [17].

Whilst the third‐trimester transvaginal scan is being performed, colour Doppler can be used to interrogate the area within the chorionic membrane overlying the internal cervical os. This is a very sensitive means of identifying vasa praevia, with the potential to prevent fetal mortality and morbidity following spontaneous rupture of membranes and rapid exsanguination [18]. There is some debate as to whether all women should be routinely screened for vasa praevia, which has a prevalence of 1 in 1500 to 1 in 2000 pregnancies; whilst routine screening may identify most cases, there is the potential for false positives that may be harmed through a policy of preterm abdominal delivery [19]. The positive predictive value of screening will be low (due to the low disease prevalence), so the prospect of the risk of harm needs careful prospective assessment. An alternative strategy would involve routine transvaginal assessment for vasa praevia in women deemed to be at high risk; this can be defined through maternal characteristics such as advanced maternal age, in vitro fertilization (IVF) pregnancy, known placenta praevia or velamentous cord insertion [20]. Screening for vasa praevia may be performed at 20 weeks rather than in the third trimester but there are few prospective data comparing the efficacy of this screening tool at these two time points.

Cervical length can also be used to predict the potential for success of induction of labour. Induction is commonly offered to some cohorts, for example diabetic women, due to recognized increased risks of perinatal mortality. A transvaginal scan may be used to predict the likely success of induction and could potentially be used to guide the style of induction, although there are currently no prospective data demonstrating that this improves outcomes [21]. The findings of a transvaginal/transperineal assessment can be combined with measures of fetal biometry and other maternal characteristics to predict the likelihood of emergency caesarean section [22]. Some researchers have suggested that this could be used to define a cohort of women that should be offered an elective caesarean section, but others believe that the predictive value of the test is not sufficient to merit such significant intervention.

Women who have had a previous caesarean section, or other uterine surgery, have an increased risk of placenta accreta. The risk of placenta accreta is associated with the number of previous caesarean sections [23]. Transabdominal ultrasound can be used to predict placenta accreta and percreta, and may be used in conjunction with MRI to define the level of risk and extent of extrauterine invasion [24]. Thinning and disruption of the hypoechoic interface between the placenta and myometrium can often be seen using conventional two‐dimensional grey‐scale imaging. The placenta may include large lacunae and these, together with other aspects of increased and chaotic myometrial/placental blood flow, may be identified using colour Doppler.

There are also some data describing the value of measuring myometrial thickness for the prediction of risk of scar dehiscence in women who have had a previous caesarean section [26,27]. This is a difficult measure to make reliably as there are limitations to approach using both transabdominal and transvaginal methodologies. Various cut‐offs for defining a high‐risk cohort have been described and there are no data to demonstrate improved perinatal outcomes by screening all women who have had a previous caesarean section during the later part of the third trimester [27]. Although there is continued research interest in screening this cohort of women, it is probably premature to suggest this should be routine clinical practice.

Predicting intrauterine growth restriction and intrapartum fetal hypoxia

The second commonest cause of stillbirth (excluding congenital abnormalities) is IUGR [28]. There appear to be two different phenotypes of IUGR in the third trimester [29]. In the absence of an underlying chromosomal, genetic or structural anomaly or of fetal infection, early‐onset IUGR (leading to delivery before 32 weeks) is typically an expression of placental insufficiency and is defined by demonstrating poor growth, with measures of the abdominal circumference and estimated fetal weight lying below an established (3rd, 5th or 10th) centile [30]. The process of fetal compensation then decompensation can be assessed using a number of clinical tools and this process and subsequent management is described in more detail in Chapter 17 [31]. In contrast to this, late‐onset IUGR (>34 weeks’ gestation) is associated with placental failure rather than insufficiency [32]. Consequently, the fetus may have initially maintained normal growth, and a single measure of biometry may not be sufficient to identify reduced growth velocity. Similarly, tests that are traditionally used to define and manage placental insufficiency (umbilical artery and ductus venosus Doppler) may not be of value, or may have to be used in a different way to be useful in detecting fetal compromise due to placental failure [29].

Stillbirth is a clearly defined, catastrophic but uncommon (in developed societies) end‐point. IUGR may have other more insidious impacts. Fetuses that have limited reserve are more likely to be adversely affected by uterine contraction during labour and are at higher risk of hypoxic ischaemic encephalopathy. IUGR is the most significant antenatal risk factor for the development of neonatal encephalopathy and term cerebral palsy [33]. IUGR is also strongly associated with non‐communicative cardiovascular and metabolic diseases that affect infants, adolescents and adults and therefore potentially has a lifelong impact on an individual’s health and well’being [34,35]. Appropriate identification of IUGR fetuses, through third‐trimester fetal assessment, provides the means to intervene to improve outcomes for a significant proportion of pregnancies.

There are conflicting opinions about the most appropriate strategies and methodologies that should be used to define normal fetal growth. Fetuses cannot be repeatedly removed and measured and replaced in the uterus to develop charts based on longitudinal measures and most charts for parameters such as birthweight include populations of fetuses that delivered because of some inherent abnormality. Charts based on data derived from such populations have centiles that are too low, particularly at early gestations, where there is also increased inaccuracy due to smaller numbers of cases [36]. Other researchers have focused on the issues of population diversity and have suggested that all charts should be customized according to a mother’s own characteristics [37]. A third approach, based on the observation that ‘perfect’ subgroups of women from a range of ethnic groups have similar patterns of growth, suggests that we should use international standardized charts to reduce heterogeneity [38]. There are no data to show whether use of customized or standardized charts improves perinatal mortality, and it is important to recognize that intervention in fetal growth (through premature delivery) may cause harm [39,40].

The simplest approach to identifying babies that are small for gestational age (SGA), and at increased risk of IUGR and its sequelae, involves clinical assessment through abdominal palpation and measurement of symphysis–fundal height. The effectiveness of this assessment has been variously reported but in one large trial involving more than 6000 women had a low sensitivity (28%) at 95% specificity [41]. Whilst single measures have poor predictive value (7% in this trial), this improves with serial measurement and through the use of customized charts, although there are no prospective studies that have proven improvement in perinatal mortality and morbidity [42].

Ultrasound can be used to assess fetal biometry, and measures of the biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC) and femur length (FL) are typically used to estimate fetal weight (Fig. 18.2) [38,43]. Imaging and measuring these structures becomes more difficult as the third trimester advances and the sensitivity, specificity and positive and negative predictive values have been described as 53% (95% CI 49–58%), 81% (95% CI 80–83%), 26% (95% CI 23–29%) and 93% (95% CI 93–94%), respectively [44]. In addition, assessment of single measures of biometry will not identify previously well‐grown fetuses that have been exposed to a later insult and are now crossing centiles. In a study involving 4000 pregnancies, Sovio et al. [45] showed that the routine application of ultrasound for serial growth assessment improved detection of SGA nearly threefold when compared with routine clinical management (57% vs. 20% in those that had a ‘selected’ ultrasound). Further improvements might be seen though computerized analysis of growth velocity and a number of groups have developed different methodologies for this process of risk assessment [46,47]. Implementation of a programme of serial ultrasound assessment to monitor fetal growth has not been tested to establish whether it leads to an improvement in perinatal mortality and morbidity. Given the potential for harm through preterm delivery and the significant expense of adding serial scans to routine care, it would be important to demonstrate the added value prior to implementation.

Improvement of the outcomes of fetuses with late‐onset IUGR first requires better detection of affected fetuses. Given the difficulties of assessing fetal biometry, other measures that show functional changes in growth‐restricted fetuses may be of value in improving screening efficiency. The simplest of these tools involves the mother, by asking her to monitor fetal movement and to report reduced fetal movement in a timely fashion. A large randomized controlled trial conducted in the 1980s failed to demonstrate any value in self‐reporting of reduced fetal movement, but the quality of these data is disputed as there was (amongst other issues) a significant improvement in outcomes in the non‐treatment arm as well as in women randomized to assessment of fetal movement [48]. More recent work has suggested that maternal assessment of fetal movement is important and that perinatal mortality is lower amongst groups that are counselled about self‐monitoring [49]. Outcomes may also be improved through protocol‐driven management of women who attend with reduced fetal movements, often through inclusion of investigational tools such as cardiotocography (CTG) or targeted ultrasound assessment [50].

The Perinatal Society of Australia and New Zealand (PSANZ) guideline for management of reduced fetal movement recommends that CTG should be performed for all women who present with reduced fetal movements during the third trimester of pregnancy [51]. CTG provides continuous graphical representation of the fetal heart rate together with information about uterine activity. CTG can be used in a variety of circumstances including intrapartum monitoring as well as for antenatal fetal assessment. The interpretation of a CTG depends on clinical circumstance, specifically whether a woman is defined as being in labour. Consequently, it is extremely important that an appropriate history and examination is performed before CTG can be interpreted; our anecdotal experience has shown that inappropriate interpretation of antenatal CTG in women who present with reduced fetal movements and some uterine activity, but who are not in active labour, can lead to false reassurance of clinicians and poor pregnancy outcomes.

There is limited evidence demonstrating that antenatal CTG improves perinatal outcome but the tool is commonly used and is generally freely available within the antenatal assessment environment, so traces are easy to procure [52]. Difficulties in application lie in interpretation. Some aspects, such as measures of baseline heart rate, acceleration, deceleration and short‐ and long‐term variability, can be defined through computer analysis and algorithms for further management can be defined on the basis of these findings [53]. In a series of 305 women presenting with reduced fetal movement during the third trimester, Dutton et al. [54] found that an abnormal CTG was the most significant finding associated with adverse pregnancy outcome, reporting an odds ratio (OR) of 7.08 (95% CI 1.31–38.18); the impacts of other clinical findings, such as diastolic blood pressure and a low estimated fetal weight centile, were much less significant.

Whilst an abnormal antenatal CTG is strongly associated with adverse fetal outcome in a woman presenting with reduced fetal movements, this may not prove the best investigation for defining fetal well‐being and making decisions about timing of delivery. The antenatal CTG that shows reduced variability, no accelerations and, on some occasions, shallow decelerations may be indicative of a fetus that is compromised by hypoxia, but these findings may be indicative of an irreversible neurological impact so delivery at this stage would be too late to effect the best outcome for this fetus. Alternatively, in some acute situations, the cardiovascular response to a hypoxic insult may be incomplete; the changes seen in baseline heart rate, variability and decelerations may only occur a few hours before death and could be missed through monitoring prior to this time.

CTG monitors fetal heart rate and uterine activity. This information may be valuable and may help define the need for delivery. CTG does not report other maternal parameters (e.g. heart rate, temperature and blood pressure) nor does it provide information about fetal size. It is therefore important to recognize the potential limitations of the technology and to interpret the findings with a good understanding of the clinical circumstances and risks. In a trial of postdates women being randomized to induction or ongoing expectant management, a reassuring CTG was associated with higher perinatal mortality; clinicians need to be aware of the risk of false reassurance that individual tests can provide [55].

In addition to assessing fetal biometry, ultrasound can be used to assess functional parameters that may be impacted by poor fetal growth. The growth‐restricted fetus may compensate by reducing renal perfusion, with consequent reduction in urine production that can be assessed through measurement of amniotic fluid levels [56]. The amniotic fluid index (measurement of the liquor in the four quadrants) is used as a surrogate measure of amniotic fluid volume and should be measured in a standardized manner, holding the ultrasound probe perpendicular to the abdomen (an oblique measure will be falsely increased) and measuring the maximal clear (no fetal parts) pool in each quadrant (Fig. 18.3). Normal amniotic fluid indices vary widely across the third trimester, with wide standard deviation [56]. The amniotic fluid index peaks at about 34 weeks’ gestation and levels typically reduce towards term. Cut‐offs of less than 5 cm and more than 25 cm are typically used to define oligohydramnios and polyhydramnios, respectively. A recent meta‐analysis that examined the association between anomalous amniotic fluid indices and adverse perinatal outcome reported that oligohydramios was associated with birthweight below the 10th centile (OR 6.31; 95% CI 4.15–9.58) as well as with perinatal mortality (OR 8.72; 95% CI 2.43–31.26), although the association did not have a strong predictive value for these outcomes [57].

A variety of different uteroplacental Doppler measures can also be assessed by ultrasound and may be of value in assessing fetal growth and well‐being. The vessel most commonly assessed in the third trimester is the umbilical artery (Fig. 18.4). In early‐onset IUGR, associated with placental insufficiency, the umbilical artery Doppler waveform typically changes with reduced forward flow in diastole and an increase in the vessel’s pulsatility index (PI). A meta‐analysis of studies that reported umbilical artery Doppler parameters in pregnancies defined as having a high risk of fetal growth restriction during the third trimester found that absent or reversed end‐diastolic flow was associated with a significant increase in risk of perinatal mortality [58]. In many centres, umbilical artery Doppler parameters are now reported as a matter of routine during a third‐trimester scan, but it is important to interpret the findings in the context of the literature, as other studies have shown that this is not a useful marker for fetal well‐being in low‐risk patient cohorts late in the third trimester [59]. Management of IUGR, placental insufficiency and the finding of an abnormal umbilical artery Doppler is addressed in more detail in Chapter 17.

Later in the third trimester, where the pathology underlying growth failure and ultimately stillbirth is more likely to be due to placental failure rather than placental insufficiency, other Doppler indices become more important in defining fetal welfare. Recent research has shown that, later in the third trimester (i.e. above 32 or 34 weeks) the best marker of fetal compensation for hypoxia is diversion of blood flow, and therefore more oxygen, to the brain. The middle cerebral artery (MCA) can be examined as a marker for this, with demonstration of reduced vascular resistance and a low PI in the fetus that is compromised but compensating effectively (Fig. 18.5) [60,61]. A number of groups have reported that a low MCA PI is associated with low fetal birthweight as well as with fetal distress and the need for emergency caesarean section in labour [62–64]. It has been suggested that presentation of Doppler data as a ratio of middle cerebral and umbilical artery PIs (the cerebroplacental ratio) improves the sensitivity and specificity of this tool in defining high‐risk pregnancies [65]. As yet, there are few data that have demonstrated that either routine screening with a cerebroplacental ratio or application to specific high‐risk groups (e.g. those with decreased fetal movements) improves pregnancy outcome but there is a lot of interest in the potential value of this tool and further trials of Doppler monitoring late in the third trimester are ongoing [66].

The ultrasound assessment of fetal growth, amniotic fluid index and haemodynamics has, to a large extent, replaced the more formal process of assessing fetal well‐being by scoring the biophysical profile. A formal biophysical profile includes four ultrasound‐based assessments – fetal breathing, movement and tone and the amniotic fluid index – which are then combined with a CTG ‘non‐stress’ test [67]. Each component of the test is scored two points (if normal) or zero points (if abnormal). Scores below 10 lead to further intensive surveillance or induction of labour depending on the level of the score and the gestation of the pregnancy. Whilst the biophysical profile provides a clear structured method of assessing fetal well‐being, it fails to include all parameters that may be of importance (biometry and Doppler) and does not adequately weight the various components of the test in relation to their likely association with pathology. The score has, on occasion, been modified and applied in various high‐ or low‐risk circumstances with various ongoing management strategies [68,69]. Despite this, there is no clear evidence of benefit compared with interpreting ultrasound and/or CTG findings through a less structured approach [70].

Predicting fetal macrosomia, risk of failure to progress and shoulder dystocia in labour

Fetal macrosomia also poses a significant risk of adverse pregnancy outcome in the third trimester of pregnancy. Macrosomia is associated with stillbirth and birth injury (shoulder dystocia, brachial plexus injury and limb fracture) as well as increased rates of operative delivery and perineal trauma [71]. Postnatally, macrosomic infants have higher rates of admission to the neonatal unit and are more likely to develop hypoglycaemia and hyperbilirubinaemia [72]. Macrosomic infants have increased rates of diabetes, metabolic syndrome and cardiovascular disease later in childhood and in adult life [73]. Across the world, changes in lifestyle and diet are contributing to increasing rates of gestational diabetes and hyperglycaemia in pregnancy and we face a global epidemic of macrosomia and its sequelae [74].

Despite this, and in comparison with IUGR, macrosomia is poorly defined and under‐researched. Macrosomia has traditionally been defined using a fixed birthweight cut‐off (commonly 4000 or 4500 g) independent of gestational age of delivery. These thresholds are based on the finding of increased rates of morbidity above these limits, although the reality is that these are continuous variables and it would likely be better to describe risks based on algorithms that define centiles in relation to, rather than being independent of, gestational age, although this adds complexity to categorization [75]. The association with stillbirth, and the underlying mechanism of death, is also poorly understood, which makes functional assessment of risk to the macrosomic fetus challenging. Many macrosomic infants are born to women who have diabetes in pregnancy and it has been suggested that stillbirth results from metabolic acidosis in these cases [76]. Interestingly, others have suggested that death might occur if the fetal cell mass exceeds placental ability for tissue oxygenation, and it has been proposed that maternal cardiac failure may contribute to fetal death in this circumstance [77].

Clinicians have a poor history for accurate estimation of fetal weight in the later part of the third trimester. In 1992, Chauhan et al. [78] reported that women predicted fetal weight more accurately than either clinical or ultrasound examination. The heterogeneity of the literature, using different charts and cut‐offs to define macrosomia, make it difficult to determine absolute sensitivity of ultrasound and studies have reported detection rates of 15–79% [79]. Researchers have compared different algorithms for estimation of fetal weight and prediction of macrosomia which have shown significant variation in findings. Predicting risk purely through ultrasound assessment using standard biometric parameters appears unlikely to be useful in defining a macrosomic cohort in routine practice [80]. There is also evidence that different algorithms should be applied to diabetic and non‐diabetic populations [81].

Risk assessment may be improved by acknowledging differing maternal characteristics and using a Bayesian multivariate approach to screening [82]. It is not clear whether screening should then be limited to high‐risk cohorts (perhaps defined through maternal characteristics and/or medical history), through a contingent process based on findings of screening tests performed at an early stage of pregnancy, or to the whole population [83].

Opinion regarding the management of macrosomic fetuses at term has typically been divided, with few prospective data to inform clinical practice. Some have argued that induction of labour may prevent maternal and neonatal morbidity associated with delivery of an overgrown infant whilst others have maintained that the process of induction carries its own inherent risks. There are now four randomized controlled trials that, in meta‐analysis, appear to show that induction of labour may be beneficial and result in reduction in rates of neonatal birth injury (brachial plexus injury and limb fracture) [84]. Some of the trials in this meta‐analysis are relatively small and did not report all outcome measures. These are also relatively uncommon outcomes, so 60 inductions need to be performed to prevent one adverse outcome. When this is coupled to the fact that we do not yet have a validated and strongly predictive screening test, it is difficult to advocate a process of routine screening and intervention without further research in this field [85].

In addition to developing algorithms that identify the macrosomic fetus, some groups have focused on the development of algorithms that will predict the likely success of induction of labour or the spontaneous onset of labour. These algorithms variously include maternal demographic factors with ultrasound estimates of fetal size and/or fetal Doppler, cervical length and/or mobility of the pelvic floor [86–89]. There is currently no consensus on the factors that should be included in such a predictive model and no prospective validation or demonstration of improved maternal and perinatal outcomes through application of such a test [90].

Conclusion

Ultrasound is a diverse tool that has many applications in the third trimester. In many settings, a third‐trimester scan is not currently part of a routine antenatal screening strategy and there are limited data to support routine population‐based screening. It is important, however, to recognize that many contemporary obstetric issues are centred around the third trimester and ultrasound is likely to play a significant role in defining an individual’s risk and ensuring that appropriate strategies for maintaining pregnancy and expediting delivery are developed. Ultrasound is often best applied as one component of multivariate risk assessment and this is the subject of much ongoing research, with applications as diverse as the prevention of stillbirth, shoulder dystocia or maternal perineal trauma. Given the current medicolegal environment, it is likely that these methods of risk assessment will become an integral part of management of the third trimester as they will allow clinicians to have better informed conversations about risk with their patients.