Table 1 Details of the Data Sources

Data used

Birthweight ≤ 40 weeks

Head and length measures ≤ 40 weeks

Head and length measures ≤ 40 weeks

Weight, head and length measures after term

Sample size (n)

676,605

376,000

Head circumference: 29090 and length: 26973

Birth data :weight: 82 million, length: 900,000, head circumference: 400. First year of life: 2200 to 38,000 infants.

n < 30 weeks

4101

0

274

N/A

Gestational age range of study

22 to 43 weeks

28.5 to 42.5 weeks

22 to 43 weeks

Post term

Inclusion criteria

All births

"Healthy" newborn infants

Singleton livebirths

NHANES surveys

Exclusion criteria

Ontario (province) was excluded due to problems with data quality.

Stillbirths, twins, complications during pregnancy with potential effects on fetal growth and significant malformations

Multiple births

Infants with birthweights < 1500 grams

Dates

1994 to 1996

1977 to 81

1982 to 1995

Primarily 1963 to 1994

Completed gestational weeks

yes

yes

yes

N/A

Method to assess gestational age

"early ultrasound has increasingly been the basis for gestational age assessments in recent years"

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.

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.

N/A

Outliers

Assumed a log normal distribution of birthweight at each gestational age and compared the probabilities of accurate versus misclassification of infant's gestational age.

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.

Curves were smoothed with a variety of parametric and non-parametric procedures