Third Trimester Ultrasound in Australia, an Academic Report
Stillbirth is a global human tragedy. In Australia, the stillbirth rate of ~7 per 1000 births (1) has remained relatively static over decades (2). This highlights the pressing need for better approaches to detecting and managing established risk factors for stillbirth. Many relevant risk factors, such as undiagnosed breech presentation in labour and fetal growth restriction, may be identified with point-of-care ultrasound examinations in the third trimester.
Better detection of breech presentation
Undiagnosed breech presentation in labour increases the risk of perinatal morbidity and mortality (3). Breech presentation at term is common, occurring in 3%–4% of pregnancies (3). Despite how easy it is to detect with bedside ultrasound, some women still enter labour with a fetus in undetected breech presentation (4). The majority of these women are then delivered via emergency caesarean section, with increased morbidity and mortality for mother and baby.
In current practice, fetal presentation is routinely assessed by abdominal palpation by a midwife, obstetrician, or general practitioner, despite this strategy only detecting 70% of cases (5). Ultrasound examination is a quick, safe and currently under-utilised method that accurately identifies fetal presentation. Importantly, for cases of breech presentation near term, effective interventions exist. The Royal Australian and New Zealand College of Obstetricians and Gynaecologists recommends that women with suspected breech presentation near, or at, term should first have this confirmed with ultrasound before being offered an external cephalic version (if appropriate) to reduce caesarean section (6). Elective caesarean section carries a lower morbidity rate than emergency caesarean section – often the mode of delivery for cases of undiagnosed breech presentation in labour. Universal late-pregnancy ultrasound to assess fetal presentation would virtually eliminate undiagnosed breech presentation in labour, with likely associated reductions in morbidity and mortality (7). There is therefore great impetus for maternity care providers to improve access to this effective antenatal strategy for the pregnant women of Australia.
Better detection of fetal growth restriction
Improved clinical detection of fetal growth restriction (FGR) due to uteroplacental insufficiency is a leading priority in antenatal care, as FGR is consistently identified as a major risk factor for stillbirth.
FGR describes fetuses who fail to reach their biological or genetic growth potential (8, 9). It most commonly reflects uteroplacental insufficiency, where the oxygen and nutrient supplies to the fetus are compromised, and this may render the fetus unable to maintain adequate growth (10, 11). Importantly, when FGR fetuses are identified during pregnancy, they are delivered earlier, and the stillbirth risk is halved (12). There is no increase in the rate of caesarean section when women with fetuses who have suffered placental insufficiency have an induction of labour at term (13). This means there is a safe and effective intervention to reduce stillbirth among pregnancies complicated by late-onset FGR. Unfortunately, antenatal detection of FGR remains poor. Improved identification of FGR has thus been recognised as a ‘top 10 priority’ to reduce the global burden of stillbirth (14).
Recently, consensus definitions for FGR have been published (8). Abnormal umbilical artery (UA) and middle cerebral artery (MCA) Doppler findings are included as a defining features. Currently, the reality is that many antenatal care providers in Australia do not feel confident in when or why these Doppler evaluations are warranted. Many also lack confidence when it comes to interpreting Doppler investigation results.
In cases of early-onset FGR an elevated UA pulsatility index (PI) is often the first Doppler abnormality seen. Reduced diastolic flow in the UA reflects increased placental resistance associated with progressive obliteration of the villous vascular tree (15, 16). Over time, abnormal UA blood flow resistance progresses to demonstrate absent and then reversed end-diastolic flow (17, 18). These abnormalities are all strongly associated with adverse perinatal outcomes (19), justifying the inclusion of abnormal UA Doppler parameters in the definition of early-onset FGR.
In contrast, abnormal UA Doppler is a more minor feature in late-onset FGR. The combination of less severe placental pathology and increasing vessel calibre in the late preterm or term placenta means that UA resistance is uncommonly elevated (20, 21). Most adverse events due to placental dysfunction in late pregnancy occur in the presence of normal UA Doppler measurements (22) despite histological evidence of placental malperfusion (23).
Instead, in late-onset FGR the fetus adapts by vasodilating the blood vessels of its brain. This cerebral redistribution predominates, often with normal UA measurements (24). Late-onset growth restricted fetuses show decreasing cerebroplacental ratio (CPR) values as gestation progresses (24), hence its inclusion in the consensus definition. The CPR is the MCA PI divided by the UA PI. It quantifies even subtle changes in fetal cerebral redistribution relative to potentially increasing placental resistance measured in the UA. Decreasing CPR values thus reflect increasing uteroplacental insufficiency, and fetal adaptation to its environment, and are followed by significantly decreasing resistance in the MCA itself (24).
This online course in third trimester obstetric ultrasound can help
Bedside ultrasound has a multitude of clinical applications in the third trimester that can potentially improve perinatal outcomes. In Australia, General Practitioners are key antenatal care providers, however many lack the knowledge base and/or training to feel confident performing third trimester ultrasound assessments. With RACGP CPD accreditation, this Third Trimester Obstetric Ultrasound course can be completed in around 12 hours, on any desktop or mobile device with iOS, Android or Windows systems. The aim of this course is to provide health practitioners with an evidence-based approach to assessing fetal presentation and wellbeing in the third trimester. It includes explanations of the relevant physiology, and clinical case examples so that practitioners understand when ultrasound assessments are indicated and why, and to aid in the accurate interpretation of results. In addition, it explains ultrasound probe and machine manipulation, common ultrasound artefacts and the steps and requirements to adequately perform each of the bedside ultrasound assessments. This course goes as far as possible in an online format to equip clinicians with the required knowledge and confidence to embark upon practical hands-on training.
You can find further information by visiting the course page here
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