Preterm birth is defined as delivery before 37 completed weeks (259 days). To accurately differentiate between preterm and term delivery it is crucial to have a reliable estimate of gestational age. Sono-graphic determination is the most accurate method to estimate gestational age. When ultrasonography is not available, gestational age can be determined by patient's recall of the time of last menstrual period, physical examination of the size of the uterus, and examination of the neonate. These methods can be used alone or in combination but are inaccurate.
Early pregnancy sonographic estimation of gesta-tional age is crucial also for estimation of fetal growth in utero, which is assessed by evaluating the size of several fetal anatomical parameters and comparing those measurements with the normal ranges at specific gestational ages obtained from reference populations with growth that can be considered unaffected by pathological conditions. Alternatively, fetal growth can be assessed by the anthropometrical evaluation of the neonate. Several classification systems have been proposed for newborn birth weight. The simplest is categorizing new-borns <2500 g as having a low birthweight, but this classification does not differentiate between infants born small for their gestational age and infants who are small because they are born preterm. Reference charts of birthweight at different gestational ages classify infants as SGA, a proxy for IUGR; adequate for gestational age; and large for gestational age. WHO defines SGA as a birth weight below the 10th percentile for a given gestational age based on the sex-specific reference by Williams et al. Because it is based on percentile distributions, this classification categorizes some normal, constitutionally small, newborns at the lower end of the normal fetal growth distribution as growth restricted. In addition, the interpretation of the reference data is complicated by inaccuracies in the estimation of gestational age at delivery and by the pathological processes that may affect the size of infants born early in gestation.
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