Table 1. Measurements and threshold used for size-at-birth definitions.

Risk factors (exposures)	Measurement units and thresholds used in definitions
Continuous measures
Gestational age (GA)*	The duration of gestation is usually reported in completed weeks with additional days, or in completed days.
Birth weight (BW)†	Weight at birth measured in gram or kg. Reported using birth weight thresholds below or as mean birth weight with standard deviation
Small size at birth
Extremely preterm (EPT)	<28 gestational weeks
Very preterm (VPT)	<32 gestational weeks
Preterm (PT)	<37 gestational weeks
Extremely low birth weight (ELBW)	<1000 g
Very low birth weight (VLBW)	<1500 g
Low birth weight (LBW)	<2500 g
Small for gestational age (SGA)	<10th percentile of birth weight for GA
Intrauterine growth restriction (IUGR)	Defined in the footnotes of online supplemental material 3 tables 1 a-g
Large size at birth/post term
Post term	>41 gestational weeks
High birth weight (HBW)/macrosomia	>4000 g
Large for gestational age (LGA)	>90th percentile of weight for GA

^*GA is counted in calendar days from the first day of gestation, with the number of completed weeks calculated as the number of days divided by 7, presented as a whole integer plus a remainder, for example, day 258 is 36+6. Methods used to assess GA vary by study, which can affect reliability and comparability between studies. Methods using ultrasound assessment in the first trimester are most accurate.

^†Birth weight is the first weight of the fetus or neonate obtained after birth. For live births, birth weight should preferably be measured within the first hour of life before significant postnatal weight loss has occurred.

GAgestational age