Usefulness perinatal prognosis assessment using z-score over current low birth weight metric: a descriptive study

Background : A cut-off weight of 2500 g in infants, which is commonly referred as low birth weight infants, was established as an indicator of perinatal prognosis several decades ago. However, perinatal medicine developed dramatically since then and theoutcomes are discussed using other factors including fetal growth nowadays. This study was conducted to assess the present perinatal situation with recent assessment scales and reconsider the indicators used for perinatal outcome. Methods : This is a retrospective descriptive study analyzing whole Japanese birth certificate data between 1992 and 2018. Trend in prenatal outcomes including birth weight, gestational age, presence of fetal growth restriction, preterm birth and birth weight-to-gestational age z-score were calculated. Fetal growth restriction was defined as birth weight-to-gestational age z-score less than 10 percentile. Results : A total of 48,235,783 births in Japan were included. The proportion of infants with birth weight under 2500 g was constant while mean birth weight stopped declining and mean gestational age continued to fall. In contrast, the incidence of fetal growth restriction and the mean birth weight-to-gestational age z-score were improving. In detail, proportion of early term infants (between 37 weeks 0 day and 38 weeks 6 days of gestational age) was increasing while that of late preterm infants (between 34 weeks 0 day and 37 weeks 6 days of gestational age) was constant. Conclusions : Over all perinatal outcome was improving. Shortened gestational age result from change in perinatal management policy due to progress in perinatal medicine that enabled smaller infants and/or infants born at shorter gestational age to survive. However, even if delivered with a shorter gestational period with an aim to achieve good outcomes, considering that the 10th percentile weight of a baby boy born to a primiparous mother at 37 weeks 0 days gestation, a full-term delivery, is 2203 g, for example, the infants would be classified as a low birthweight infant which is an indicator for poor prognosis. Therefore, low birth weight no longer reflects perinatal outcome appropriately and other factors like fetal growth should be considered as indicators for overall perinatal context.