Abstract
BackgroundFully understanding the determinants and sequelae of fetal growth requires a continuous measure of birth weight adjusted for gestational age. Published United States reference data, however, provide estimates only of the median and lowest and highest 5th and 10th percentiles for birth weight at each gestational age. The purpose of our analysis was to create more continuous reference measures of birth weight for gestational age for use in epidemiologic analyses.MethodsWe used data from the most recent nationwide United States Natality datasets to generate multiple reference percentiles of birth weight at each completed week of gestation from 22 through 44 weeks. Gestational age was determined from last menstrual period. We analyzed data from 6,690,717 singleton infants with recorded birth weight and sex born to United States resident mothers in 1999 and 2000.ResultsBirth weight rose with greater gestational age, with increasing slopes during the third trimester and a leveling off beyond 40 weeks. Boys had higher birth weights than girls, later born children higher weights than firstborns, and infants born to non-Hispanic white mothers higher birth weights than those born to non-Hispanic black mothers. These results correspond well with previously published estimates reporting limited percentiles.ConclusionsOur method provides comprehensive reference values of birth weight at 22 through 44 completed weeks of gestation, derived from broadly based nationwide data. Other approaches require assumptions of normality or of a functional relationship between gestational age and birth weight, which may not be appropriate. These data should prove useful for researchers investigating the predictors and outcomes of altered fetal growth.
Highlights
Understanding the determinants and sequelae of fetal growth requires a continuous measure of birth weight adjusted for gestational age
In 1996, Alexander et al published birth weight curves using a 1991 nationwide United States reference, later updated using 1994–1996 data.[10,11]. These authors included weights only for the 5th, 10th, 50th, 90th, and 95th percentiles at each completed week of gestation. While such categorical divisions may serve for clinical or demographic use, many research studies have been hampered by the traditional classifications "small for gestational age" (SGA) and "large for gestational age" (LGA), historically defined as those infants below the 10th or above the 90th percentile at each gestational age.[12]
We obtained data from the National Center for Health Statistics 1999 and 2000 Natality Data Sets on CDROM.[14]. These public-use data files include information recorded on birth certificates from all 3,963,465 live births in 1999 and 4,063,823 live births in 2000 that occurred in the United States
Summary
We obtained data from the National Center for Health Statistics 1999 and 2000 Natality Data Sets on CDROM.[14]. We used a resistant nonlinear smoothing technique, 4325H,[15] twice, to account for the same birth weight value straddling several percentiles within each week of gestation, which might represent a bias towards reporting round numbers of birth weight values. This approach differs from that used by Alexander, et al.,[10] who smoothed across gestational age groups. In comparison with the reference group, this infant is lighter than it would appear using a normal distribution assumption In contrast to this usual practice, we first calculated actual percentile ranks, and assigned z-values for each percentile. We performed all analyses using SAS version 8.2 (SAS Institute, Cary NC), with the exception of the smoothing procedure, for which we used STATA version 7 (STATA Corporation, College Station TX)
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