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Reference standards for fetal weight and birth weight at various gestational ages are better based on ultrasound estimations of fetal weight than on birth weight because infants born early are more likely to be growth retarded and therefore to skew the percentile charts. Reference standards based on ultrasound estimates of fetal weight may be non-customised, customised (or individualised) by adjusting for ethnic origin, maternal factors such as height and weight, parity and fetal sex, or made country-specific by adjusting for local data, most conveniently average birth weight at term. Non-customised standards are based on women of European stock and individualised standards are based on various ethnic groups in developed countries; neither is applicable to developing countries. Now (Lancet 2011;377:1855–61; see also comment, ibid: 1812–4) country-specific reference standards have been derived by adjusting for local mean birth weight at 40 weeks. Data from 24 countries in Africa, Latin America and Asia, taken from the 2004–08 WHO Global Survey on Maternal and Perinatal Health report, were used for application and validation of the new standards. The country-specific standards were accurate in identifying fetuses and neonates at increased risk of stillbirth, or neonatal mortality or morbidity. The OR for adverse perinatal outcome (‘small for gestational age’ vs ‘not small for gestational age’ fetuses and neonates) using non-customised, individualised or the new country-specific reference standards were 1.59, 2.84 and 2.87, respectively. The new reference standards are easily adapted to …

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