Defining small for gestational age
The importance of gestational age estimation
In order to overcome the problem of the definition of LBW capturing fetuses with suboptimal growth and preterm birth, the definition of SGA is used.
This presupposes accurate gestational age estimation, an essential component of pregnancy management. This not only allows appropriate scheduling of a woman's antenatal care but also informs obstetric management decisions—for example, whether administration of prophylactic corticosteroids for fetal lung maturity and transfer to another healthcare setting is appropriate in cases of preterm labour; or, at the other extreme, determines if labour induction should be scheduled in the post-term periodKnowledge of gestational age is also integral in the correct interpretation of clinical or ultrasonographic fetal growth assessment: abnormal fetal growth patterns such as SGA or LGA may be missed or incorrectly diagnosed if gestational age is unknown or incorrect. Accurate dating is not only important in individual pregnancy care, but also at population and health policy level: the lack of accurate gestational age estimation, in particular in those areas of the world at greatest risk of these conditions, means that preterm birth and SGA rates are merely estimates in much of the world—it is thought that preterm birth rates in over half of all births worldwide depend upon modelled data and are of uncertain accuracy (6) while estimates of SGA are based on the proxy of LBW (of human anthropometry (3). Standards describe growth under near-optimal conditions, and are prescriptive: they demonstrate how growth should be, rather than descriptive as reference charts are, which describe how growth has occurred at a particular time or place.
The concept of growth standards has been widely accepted in paediatrics, following the landmark publication of the findings of the WHO Multicentre Growth Reference Study (MGRS) in 2006 (26).
This longitudinal, international study followed the growth and development of children born in cities in six countries with arguably distinct ancestral lineages around the world (Brazil, Oman, Norway, United States, India, and Ghana). The study demonstrated that when mothers were free from complications in pregnancy, lived in non- socially deprived settings, did not smoke, and exclusively breastfed the infants for the first 6 months of life, the growth of their children was remarkably similar. In 2006, the WHO Child Growth Standards were released, defining optimal growth trajectories for children everywhere. Definitions of poor growth, including the clinical conditions of stunting and wasting, are thereby near universally accepted. The WHO Child Growth Standards have been adopted in over 130 countries (27). International standards have been shown to be useful both at an individual level screening for problems of growth, as well as at the population and global level to identify and compare nutrition around the world. With advancing socioeconomic progress, the population-level distribution of childhood growth gradually shifts closer to the optimal population (28).Until recently, lack of knowledge about of the optimal growth of babies in utero and preconceived ideas about fetal growth in relation to maternal characteristics have prohibited similar comparisons in the perinatal period.