Table 1.

Details of the data sources

	Voight, 2010	Olsen, 2010	Kramer, 2001	Roberts, 1999	Bonellie, 2008	Bertino, 2010	WHO, 2006
Data source	German Perinatal Survey	Pediatrix Medical Group hospitals	Canadian national file	Australian National Perinatal Statistics Unit	Scottish maternity data collection	Italian Neonatal Study	WHO multicentre growth reference study
Sample size	2,300,000	130,111	676,605	734,145	100,133	45,462	882
n < 30 weeks	14146	11377	3247	3193	2053	623	N/A
Lowest gestational age	22	23	22	20	24	23	term
Dates	1995 to 2000	1998 to 2006	1994 to 1996	1991 to 1994	1998 to 2003	2005 to 2007	1997-2003
Data	Weight	Weight, head, length	Weight	Weight	Weight	Weight, head, length	Weight, head, length
Exclusion criteria	None stated, included both live and stillborn	Multiple births, congenital anomalies, death before discharge, outlier measures (> 2 x interquartile range below the first and 3rd quartile).	Ontario province was excluded due to problems with data quality.	Omitted multiple and still births (births < 400 grams did not need to be recorded)	Multiple births, lethal anomalies, weights < 250 grams, and outlier measures (> 2 x interquartile range outside the first and 3rd quartile).	Multiple births, stillbirths, major congenital anomalies, and fetal hydrops	Maternal smoking, not breastfeeding, solids before 4 months. Screened for environmental or economic constraints.
Method to assess gestational age	Ultrasound assessment 8–14 weeks and Naegle’s rule.	Neonatologist assessment	“early ultrasound has increasingly been the basis for gestational age assessments in recent years”	Dates, prenatal, or postnatal assessment	Clinician assessment based on ultrasound, maternal dates, and clinical estimates	Ultrasound assessment first trimester	Not stated
Outliers/smoothing method	Cubic regression, LOESS smoothing, LMS parameter smoothing	LMS methods, with the skew set to one and further manual smoothing	Assumed a log normal distribution of birthweight at each gestational age and compared the probabilities of accurate versus misclassification of infant’s gestational age	Omitted outlier measures (> 2 x interquartile range below the first and 3rd quartile).	Cubic spline fitting	Generalized logistic functions	Omitted outliers > 3 SD, LMS parameter smoothing, skew set to one for weight, cubic spline fitting.