A nearly continuous measure of birth weight for gestational age using a United States national reference

Abstract

Background: Fully 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.

Methods: We 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.

Results: Birth 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.

Conclusions: Our 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.

Figure 1

Select reference percentiles for birth weight at each gestational age from 22 to 44 completed weeks for all singleton infants: 10^th, 50^th, and 90^th percentiles from our analysis of 6,690,717 births in the 1999–2000 US Natality datasets, compared with the corresponding percentiles published by Alexander et al. (1999) from 1994–96 US Natality data.

Figure 2

Select reference percentiles for birth weight at each gestational age from 22 to 44 completed weeks for male and female singleton infants: 10^th, 50^th, and 90^th percentiles. Data from 3,423,215 male and 3,267,502 female infants in the 1999-2000 US Natality datasets.

Figure 3

Select reference percentiles for birth weight at each gestational age from 22 to 44 completed weeks for firstborn and non-firstborn singleton infants: 10^th, 50^th, and 90^th percentiles. Data from 2,755,841 firstborn and 3,917,426 non-firstborn infants in the 1999–2000 US Natality datasets.

Figure 4

Select reference percentiles for birth weight at each gestational age from 22 to 44 completed weeks for infants born to non-Hispanic white and black mothers: 10^th, 50^th, and 90^th percentiles. Data from 3,979,490 infants of white mothers and 954,021 infants of black mothers in the 1999–2000 US Natality datasets.