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Data used
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Birthweight ≤ 40 weeks
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Head and length measures ≤ 40 weeks
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Head and length measures ≤ 40 weeks
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Weight, head and length measures after term
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Sample size (n)
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676,605
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376,000
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Head circumference: 29090 and length: 26973
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Birth data :weight: 82 million, length: 900,000, head circumference: 400. First year of life: 2200 to 38,000 infants.
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n < 30 weeks
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4101
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0
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274
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N/A
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Gestational age range of study
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22 to 43 weeks
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28.5 to 42.5 weeks
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22 to 43 weeks
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Post term
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Inclusion criteria
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All births
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"Healthy" newborn infants
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Singleton livebirths
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NHANES surveys
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Exclusion criteria
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Ontario (province) was excluded due to problems with data quality.
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Stillbirths, twins, complications during pregnancy with potential effects on fetal growth and significant malformations
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Multiple births
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Infants with birthweights < 1500 grams
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Dates
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1994 to 1996
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1977 to 81
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1982 to 1995
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Primarily 1963 to 1994
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Completed gestational weeks
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yes
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yes
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yes
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N/A
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Method to assess gestational age
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"early ultrasound has increasingly been the basis for gestational age assessments in recent years"
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Last menstrual period was used when it was in agreement (+/- 2 weeks) with the obstetric assessment. If not, the latter was used. When either estimate was missing the gestational age was considered unknown.
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In the majority of cases the gestational age was based on first trimester ultrasound, or when early ultrasound was not available, last menstrual period dates were used. In < 1% of cases, the Ballard assessment was used when neither dates nor ultrasound were available.
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N/A
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Outliers
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Assumed a log normal distribution of birthweight at each gestational age and compared the probabilities of accurate versus misclassification of infant's gestational age.
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The mother's medical record was examined and following verification of the gestational age and other data, outliers were either accepted as real or corrected.
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Curves were smoothed with a variety of parametric and non-parametric procedures
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