BACKGROUND: Neonatal anthropometric charts of the distribution of measurements, mainly birth weight, taken at different gestational ages are widely used by obstetricians and pediatricians. However, the relationship between delivery mode and neonatal anthropometric data has not been investigated in Japan or other countries. METHODS: The subjects were selected from the registration database of the Japan Society of Obstetrics and Gynecology (2003-2005). Tenth centile, median, and 90th centile of birth weight by sex, birth order, and delivery mode were observed by gestational age from 22 to 42 weeks among eligible singleton births. RESULTS: After excluding 248 outliers and 5243 births that did not satisfy the inclusion criteria, 144,980 births were included in the analysis. The distribution of 10th centile curves was skewed toward lower birth weights during the preterm period among both first live births and second and later live births delivered by cesarean section. More than 40% of both male and female live births were delivered by cesarean section at 37 weeks or earlier. CONCLUSIONS: The large proportion of cesarean sections influenced the skewness of the birth weight distribution in the preterm period.
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