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Sir,

Chard, Yoong & Macintosh begin their commentary (December 1993) with “fetal growth retardation is one of the commonest diagnoses made in obstetric care”, but what is being made is not a diagnosis but an observation that the baby is small for its gestational age. Being small for gestational age (SGA) must not be confused with being intrauterine growth retarded (IUGR). SGA is a descriptive term, indicating only that a baby is smaller than other babies of the same gestation in its population, which might be defined by infant gender, maternal height, race and parity as well as geographical area. SGA therefore does not necessarily signify any aetiology, pathology or outcome and hence is not a diagnosis.

By contrast, IUGR implies that the baby's birthweight is less than optimal. Optimal birthweight is not necessarily realisation of genetic growth potential. There is some evidence that a baby's genetic growth potential is seldom completely realised due to maternal constraints necessary for mother and fetus to survive delivery (Ounsted & Ounsted 1973). Not having an optimal birthweight implies pathology (the essence of a diagnosis) which may be associated with suboptimal outcome. To make the diagnosis of IUGR, both birthweight and optimal birthweight must be known, but optimal birthweight is not measurable. To assess whether an infant is IUGR, observers have substituted the following observations, in decreasing order of validity:

  • 1
    A break or faltering of fetal growth as measured by serial ultrasound or fundal height measurements;
  • 2
    For late onset of asymmetric IUGR, evidence of recent loss of adipose tissue, sometimes inferred from ratio of length to abdominal circumference, but perhaps more reliably inferred from clinical signs of recent subcutaneous tissue loss at delivery, such as a wrinkly skin with a deficit of adipose tissue;
  • 3
    The relation of birthweight for gestational age compared with that of siblings in the absence of persistent pathological growth retarding factors affecting all pregnancies.
  • 4
    The relation of birthweight for gestational age relative to that of the source population: that is, population birthweight centile which is frequently expressed as a dichotomy, SGA or not-SGA.

The fourth observation is the least valid, the most easily available and therefore used most often. However, SGA is only associated with IUGR, which means that while the group of SGA infants includes a higher proportion of IUGR infants than will be found in appropriately grown (AGA) or large for gestational age (LGA) infants, some SGA infants will have attained their optimal birthweight and some AGA and LGA infants may have failed to reach their optimal birthweight. The use of SGA as a surrogate measure for IUGR should be confined to statistical (epidemiological) studies from which we can infer a worse outcome in terms of mortality (Battaglia & Lubchenco 1967) or spastic cerebral palsy (Blair & Stanley 1992) from among those who are IUGR than among those who are not. However, we have no means of knowing which individuals are IUGR simply from their birthweight for gestational age percentile position (Blair & Stanley 1992, p. 99).

While Chard, Yoong and Macintosh demonstrate an appreciation of the distinction between IUGR and SGA, the two terms are then confused by their definition of IUGR in terms of birthweight centiles. In contrast, Battaglia and Lubchenco (1967) do not use the term IUGR.

If the distinction between SGA and IUGR is rigorously maintained, the problems raised by Chard, Yoong and Macintosh disappear. The proportion of SGA in a population is fixed by definition. The proportion of IUGR (if it can ever be defined as a dichotomous variable) will vary with the health of that population and may even be zero in genetically normal infants born at term to healthy mothers following normal pregnancies. However, since our observation of gestational health is far from perfect and genetic screening neither all-encompassing nor universal, the perfect pregnancy can never be assured. Thus the possibility of IUGR at term must always be considered.

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