Chang, Tou Choong; (1994) Diagnosis of growth retardation in small fetuses: serial ultrasound assessment of abdominal circumference and fetal weight. Doctoral thesis (M.D), UCL (University College London).

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Abstract

The aim of this work was to evaluate the ability of serial ultrasound measurements of abdominal circumference (AC) and estimates of fetal weight to diagnose intrauterine growth retardation (IUGR) in a group of small fetuses delivered at term. Factors innuencing study design 1. The interpretation of serial ultrasound values of AC and estimated fetal weight (EFW) required appropriately derived reference standards for these measurements. 2. To determine the minimum interval between ultrasound assessments, AC and EFW were subjected to tests of reproducibility. 3. As there was no accepted method of quantifying serial values of AC and EFW, different statistical methods of describing serial data were tested against neonatal morphometric indices of "wasting", the "gold standard" used to define IUGR. Methods 1. To construct reference standards for AC and EFW, four different classes of formulae were fitted to the longitudinal data of 67 normal fetuses by least squares fitting.2. The intra-observer and inter-observer reproducibility of AC and EFW were evaluated using one-way analysis of variance and limits of agreement respectively. 3. Three different methods of quantifying serial ultrasound data were evaluated in their ability to predict a reduced neonatal ponderal index, mid-arm circumference / head circumference ratio and subscapular and triceps skinfold thickness in 104 small fetuses (defined as an AC < 10th centile in the third trimester of pregnancy). These different statistical measures were compared using receiver operating characteristic (ROC) curves. 4. The best method of quantifying serial values of AC and EFW was compared with umbilical artery pulsatility index (PI) and single estimates of fetal size in the prediction of abnormal neonatal morphometry using ROC curves. 5. The study group was divided by their antenatal growth profile into those who were normally grown and those with IUGR. Perinatal morbidity and biochemical indices of lUGR were then compared in these two groups. Results 1. Reference standards for AC and EFW were constructed using a log quadratic formula; log10(AC) = a + b (GA) + c (GA)2. The fitting of this formula to the longitudinal AC (and EFW) data for each fetus resulted in the smallest residual fitting errors and did not systematically over- or under-estimate the final values of log10(AC) [or log10(EFW)]. 2. The tests of reproducibility suggested a minimum interval between ultrasound assessments of two weeks. 3. The best method of quantifying serial values of AC and EFW was found to be a change in standard deviation scores (A SDS) between the first and last ultrasound assessment The SDS for AC (or EFW) at each ultrasound assessment was calculated as follows: SDS = (Measured AC - Mean AC) / Standard deviation of AC. 4. This method of quantifying serial AC and EFW data was superior to umbilical artery PI and the last estimate of fetal size prior to delivery in the prediction of abnormal neonatal morphometry and perinatal morbidity associated with IUGR. 5. Separation of small fetuses on the basis of their A SDS values resulted in two groups with some differences in perinatal morbidity and biochemical indices of IUGR. Conclusion In a group of small fetuses, serial ultrasound assessment of AC and EFW predicted subsequent neonatal morphometry indicative of IUGR. The method of quantifying fetal growth described in this study was useful in separating small fetuses into those with IUGR and those with normal growth.

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