Diagnosis and management of fetal growth retardation.

What is fetal growth retardation? Fetal growth retardation is best defined as the failure of a fetus to achieve its growth potential. ' Though this is the theoretically correct definition, in practice various less precise criteria have been used, including: measures of absolute size such as low birth weight (less than 2500 g), and very low birth weight (less than 1500 g); and measures of relative size (small for gestational age), variably defined as less than the third centile, less than the tenth centile, and less than two SD below the population mean for gestational age and sex. These practical criteria have limitations, and comparison of studies using different definitions of fetal growth retardation is difficult. Because normal fetal growth is under intrinsic and extrinsic control by, for example, genetic, endocrinological, and nutritional factors, there is considerable variation in the incidence of pathological fetal growth depending on the population studied.2 In addition, not all 'small' babies by these definitions are pathologically grown, and conversely, some babies that are 'not small' may not have achieved normal pathological growth. Finally, the confounding effects of preterm birth are not easy to disentangle. There is conflicting evidence as to whether the fetus that is small for gestational age has a higher mortality than the fetus that is appropriately grown when one allows for confounding variables such as congenital abnormality, intrauterine viral infection, and gestational age.3' The association between fetal growth retardation and mortality is not yet established and the question will only be resolved if sufficiently large numbers are analysed to take account of these variables. Clearly there will be increased morbidity in fetuses that are small for gestational age with either a congenital anomaly or an intrauterine viral infection, the exact prognosis depending on the diagnosis. Even when these problems are excluded, however, in contrast to the mortality data, the fetuses that are small for gestational age do seem to be at increased risk of both short and long term morbidity.3 ' When one allows for confounding variables such as gestational age, race, gender, and socioeconomic group, there may be as much as six times the risk of neurodevelopmental handicap at one year of age in these infants compared with these infants who have grown appropriately.3 In addition, the effects may vary with the severity,4 the duration,5 and the type of fetal growth retardation, namely the degree of retardation of fetal head growth.6 Which fetus is at risk of developing growth retardation? A number of epidemiological factors are associated with an increased risk of a fetus being small for gestational age. These include fetal sex, multiple pregnancy, birth order, socioeconomic group, maternal height and weight, ethnic group, and interpregnancy interval.7 Several authors have used these associations to try and predict the risk of the subsequent occurrence of a fetus that is small for gestational age. For example, Galbraith et al in a study of 8030 babies,8 claimed a positive predictive value of their risk scoring method of 9-8% and a false negative rate of 2-3% (sensitivity 69 1%, specificity 67T1%). Such risk scores have not been widely implemented; they are cumbersome to use, they have a poor predictive value, and they do not always predict the fetus that is pathologically small for gestational age. A more practical approach is to identify pregnancies with specific risk factors for a fetus being small for gestational age compared with the normal population, and then to measure the growth of those fetuses serially with ultrasound to identify those that actually develop pathological growth. The following are examples of such risk factors. Severe maternal malnutrition resulting in reduced energy intake and pathological fetal growth is seen during major famines, but is only likely to be encountered in developed countries if the mother develops a rare condition such as pancreatitis, active inflammatory bowel disease, or an eating disorder, or has a bowel resection. In such cases restricting the mother's dietary intake to less than 3-77 MJ/day for six months will result, on average, in a fetus that is 240 g lighter than the fetus whose mother ate normally.9 Alcohol consumption in pregnancy is a cause of fetal growth retardation. For example, the odds ratio for a fetus being small for gestational age if a mother takes two drinks a day is 1 62 (95% confidence interval (CI) 1-26 to 2 09). This risk rises progressively with the number of drinks consumed.7 Mothers addicted to opiates have five times the risk of fetal growth retardation compared with controls. '0 Essential hypertension alone is not associated with a greater likelihood of a fetus being small for gestational age, though this is contested by some authors.9 The risk of a fetus being small for gestational age is doubled if the mother has proteinuric pre-eclampsia alone, and quadrupled if this is superimposed on pre-existing hypertension. A mother with cyanotic cardiac disease has a risk of a fetus being small for gestaBristol Maternity Hospital, Southweli Street, Bristol BS2 8EG