Small for Gestational Age: Causes and Consequences

Abstract

The adequacy or otherwise of fetal growth in human pregnancy is of great concern to the obstetrician because of its link not only with stillbirth but also with subsequent development of the baby into adult life. Indeed, in the last 20 years, the work of the Southampton group led by David Barker (supplemented by researchers in many other centres) has led to a fuller understanding of fetal origins of adult disease. At present, we are unable to significantly modify fetal growth but we can intervene to deliver the baby from a deteriorating intrauterine environment. Without knowing more about the mechanisms of fetal growth, we will never be able to develop effective therapies to normalize it. Accordingly, it is timely that the editors Kiess, Chernausek and Hokken-Koelega have brought together 12 chapters over 167 pages to review this topic. Their book was published in hardback in early 2009 by Karger at the fairly hefty cost of £95.41. The format and cost mean that this is likely to be a book acquired for the departmental library and it is certainly not a light read. Part of the problem with this topic is that the terminology is far from standardized or used consistently. Probably the most useful definition of fetal growth restriction (we used to call it fetal growth retardation but this led to parents thinking that the baby would be brain damaged and so the terminology was changed) is failure to reach the genetic growth potential of the fetus. Even this definition is problematic because it seems likely that fetuses can develop normally within a range of growth velocities to suit the prevailing environmental conditions. Moreover, genetically abnormal babies are often small and function poorly but this is because their genetic potential is abnormal. In any case, we cannot at present assess accurately the genetic growth potential of an individual baby. Accordingly, it is more usual to classify babies according to their centile position of weight for gestational age at birth. However, even the centiles used to define smallness vary (from the 10th centile to as low as the third centile) and sometimes two standard deviations below the mean is used (the 2.7th percentile). A more dynamic way of assessing fetal growth is to monitor fetal size sequentially through pregnancy using ultrasound. A baby that is ‘falling down the centiles’ is likely to function poorly and to exhibit the signs of intrauterine ‘starvation’ after birth, i.e. reduced subcutaneous fat, small liver size, large head to body ratio (known as brain sparing), etc. One of the problems with this book is that the chapters are not consistent in the way that they define growth restriction and it is acknowledged in the introduction that the expression ‘intrauterine growth restriction is frequently used synonymously with small for gestational age’, although we know that it should not be. Thus, some of the chapters refer primarily to babies that are small for gestational age below the 10th centile (the majority of whom will be small normals), whereas others refer more appropriately to babies whose growth profiles in utero are below normal. The book is as a result more useful to those who are already familiar with such confusion of definitions and wish to use it as a valuable compendium of data and references. It will be particularly useful for students starting out in this area of research, with experienced supervisors who can guide them through the various points of view and explain the inconsistencies within the book. There is a particularly useful introduction to the molecular control of fetal growth by Steven Chernausek, as this is a topic that can be baffling in its complexity. Most of the research described relates to the developed world as in most parts of the developing world gestational age at birth is unknown and only birthweight can be measured accurately. Accordingly, the chapter describing the global perspective of the problem of poor fetal growth is restricted to describing problems associated with low birthweight (defined as less than 2.5 kg), of which the major determinant is in fact preterm birth rather than fetal growth restriction. It might have been useful if the editors had drawn together the discussions on the definition of poor fetal growth in their introductory chapter and then deleted the repeated descriptions at the beginning of each chapter, not all of which are of the same high standard and a few of which show a significant lack of insight. In summary, this will be a useful book for academic departments of obstetrics and paediatrics but cannot be recommended as a suitable introductory text for the general reader.