Fetal Growth Measurement

Abstract

Ultrasound assessment of the fetus in pregnancy has become an integral part of antenatal care. For ultrasound practitioners and pregnant women alike, it is difficult to imagine pregnancy care without ultrasound. The assessment of fetal growth by evaluating changes in serial measurements and haemodynamic assessment of the fetus with Doppler ultrasound have become valuable tools in obstetric management. Identification of pathologic fetal growth has become increasingly important as fetal growth restriction is associated with increased risk of adverse perinatal outcomes and when undiagnosed is a risk factor for stillbirth. Large fetuses may also complicate pregnancies, increasing risks for birth trauma and neonatal morbidity. With widespread adoption of ultrasound into obstetric practice, how to best discriminate between normal and pathological growth, which charts to reference measurements to, and when to perform the measurements remain contentious issues. With increasing emphasis placed on ultrasound measures of fetal size and Doppler parameters to identify abnormal growth and guide clinical management, it has become imperative that charts used for these measures are reliable and used consistently. Continued lack of standardisation has the potential for misdiagnosis of abnormal fetal growth, and the suitability of reference charts derived from one population for use on a different population is an unresolved issue and problematic in multiethnic populations. In this thesis I explore the origins of measurements used in ultrasound assessment of the fetus and how these are used to measure fetal growth and wellbeing. Ultrasound providers were surveyed to gain insights into current Australian practice in performing and reporting third trimester ultrasound. The performance of fetal reference charts currently used (now more than 25 years old) and recently published growth standards were evaluated in a prospective cross-sectional study of 1642 datasets. Lastly, methods to measure fetal growth velocity were assessed in a retrospective longitudinal study of 636 datasets. The exploration of the emergence of measurements used in ultrasound assessment of the fetus consolidates many disparate sources and provides a single account of the origin of fetal measurements. Survey findings reveal inconsistent third trimester reporting practices persist in Australia, with at least four different population based reference charts in current use and lack of uniformity in fetal Doppler assessment. Evaluation of fetal reference charts currently recommended by the Australasian Society of Ultrasound in Medicine and recently published Intergrowth 21st growth standards reveal discrepancies in the proportion of measurements falling below the 10th centile and above the 90th centile, suggesting these standards are not suitable for use in the Australian population. Assessment of fetal growth velocity between 20 and 36 weeks failed to demonstrate a strong relationship between reduced growth velocity and adverse neonatal outcome.These findings have several implications for clinical practice. Inconsistent third trimester reporting practices not only raises the possibility of false-positive and false-negative diagnosis of fetal growth restriction but also has the potential for conflicting diagnoses based on the providers' choice of reference chart. A consensus on which reference charts should be used in Australia would help remedy this situation, however evaluation of the recently published Intergrowth 21st growth standards indicates they are unsuitable for use in the Australian population. Adoption into clinical practice would result in a greater proportion of fetuses classified as large for gestational age, and fewer classified as small for gestational age. The absence of a strong relationship between reduced growth velocity between 20 and 36 weeks and adverse neonatal outcome implies formal reporting of fetal growth velocity is currently not warranted. Show more