RESUMO
Objective: To establish the reference values and growth curves of length for weight and head circumference for weight among Chinese newborns in order to provide a reference for the assessment of body proportionality at birth. Methods: A cross-sectional design was applied. A total of 24 375 singleton live birth newborns with gestational ages at birth of 24+0 to 42+6 weeks were recruited from June 2015 to November 2018 from 13 cities including Beijing, Harbin, Xi'an, Shanghai, Nanjing, Wuhan, Guangzhou, Fuzhou, Kunming, Tianjin, Shenyang, Changsha, and Shenzhen, excluding those with maternal or newborn conditions that may impact the establishment of the reference values. The generalized additive model for location, scale, and shape was employed to establish reference values in terms of percentile and growth curves of length for weight and head circumference for weight for male and female newborns. The random forest machine learning method was employed to analyze the importance of variables between the established reference values in this study and the previous published weight/length, body mass index (BMI), ponderal index (PI), weight/head circumference, length/head circumference in the assessment of symmetrical and asymmetrical small for gestational age (SGA) newborns. Results: A total of 24 375 newborns with 13 197 male infants (preterm birth 7 042 infants and term birth 6 155 infants) and 11 178 female infants (preterm birth 5 222 infants and term birth 5 956 infants) were included in this study. The reference values in terms of percentile (P3, P10, P25, P50, P75, P90, P97) and growth curves of length for weight and head circumference for weight were obtained for male and female newborns with gestational ages of 24+0 to 42+6 weeks. The median birth lengths corresponding to the birth weights of 1 500, 2 500, 3 000, and 4 000 g were 40.4, 47.0, 49.3 and 52.1 cm for males and 40.4, 47.0, 49.2, and 51.8 cm for females, respectively; the median birth head circumferences were 28.4, 32.0, 33.2 and 35.2 cm for males and 28.4, 32.0, 33.1, and 35.1 cm for females, respectively. The differences of length for weight between males and females were minimum, with the difference range of -0.3 to 0.3 cm at P50; the differences of head circumference for weight between males and females were minimum, with the difference range of 0 to 0.2 cm at P50. Based on the match between birth length and birth weight for classifying symmetrical and asymmetrical SGA, length for weight and PI contributed the most, accounting for 0.32 and 0.25, respectively; based on the match between birth head circumference and birth weight, head circumference for weight and weight/head circumference contributed the most, accounting for 0.55 and 0.12, respectively; based on the match between birth length or head circumference with birth weight, head circumference for weight and length for weight contributed the most, accounting for 0.26 and 0.21, respectively. Conclusion: The establishment of the new standardized growth reference values and growth curves of length for weight and head circumference for weight among Chinese newborns are useful for clinical practice and scientific research.