Table 4.

“Small babies”: focus on classifications and definitions.

Term	Definition	Main Problems and Limits
Small baby	A baby weighing less than 2500 g at birth	While there is a trend towards an inverse association between birth weight and clinical problems, the outcome of “small babies” depends on the pathogenesis of low birth weight; hence, measures adjusted for gestational age and growth patterns show a better relationship with short- and long-term outcomes.
Very small baby	A baby weighing less than 1500 g at birth
Pre-term delivery	Delivery before 37 completed gestational weeks; late pre-term: 34–37 gestational weeks	These three widely-used terms were defined in relation to “normal development” (at term); higher risk for mild developmental or intellectual deficits (late pre-term is mainly associated with school problems); extreme preterm is associated with higher frequency of mild deficits, and higher incidence of severe developmental neurologic problems. The risk for metabolic syndrome, hypertension and CKD, albeit less well explored, may follow a similar pattern. Once more, these definitions should be associated with data on intrauterine growth; outcome may be modified by follow-up.
Early pre-term delivery	Delivery before 34 completed gestational weeks
Very early (or extreme) pre-term delivery	Delivery before 28 completed gestational weeks
Small for gestational age baby (SGA)	A baby below the 5th or the 10th centile for gestational age, adjusted to local growth curves	This finer definition takes into account the relationship between being small or preterm and the “quality” of intrauterine growth. This definition does not distinguish between harmonic growth of genetically small children (not pathologic) and stunted growth of children not attaining their genetic growth target. Given the wide ethnic differences in growth curves, the absence of local data may impair precise interpretation. Other relevant terms are AGA (adequate for gestational age) and LGA (large for gestational age: above the 90th or the 95th centile)
Intrauterine growth restriction (IUGR)	A baby that does not reach its growth potential, i.e. one below the 5th centile for gestational age, or whose growth curve flattens in pregnancy	Growth restriction is a dynamic concept that indicates flattening of the growth curve. Data are emerging on the pivotal importance of the “quality” of intrauterine growth, which may be more important than actual birth weight, or prematurity; while babies who are SGA below the 5th centile are so often IUGR that they are included in the definitions, a baby may still be adequate for gestational age, be born at term, but be IUGR, if its growth was severely reduced in the last gestational weeks.