Contrary to popular belief, human pregnancy does not last "9 months". It normally varies somewhere from 38 to 42 weeks post conception. Any birth less than 37 weeks post conception is considered to be "premature" but that really doesn't say much. Anything less than 23 weeks is considered to be a non viable fetus almost everywhere but they are always pushing that window. 23 and 24 weekers are very dicey and if the baby lives s/he may have multiple and major life long impairments but some survive intact. 25 and 26 weekers are considered in many areas to have a fair chance of a normal life but still; many die or live but are impaired. 27 to 30 weekers can still have major problems but we are getting into a comfort zone with these babies. 32 - 33 weekers are almost easy. 34 - 37 weekers may not even go to the Neonatal Intensive Care Unit but need close follow up and have more frequent hospital readmissions.
Size is not the only issue. Babies can be AGA (appropriately sized for their gestational age) or LGA (large for their gestational age) and SGA (small for their gestational age). A SGA full term baby may superficially look like an AGA preemie but developmentally they are quite different. A LGA preemie may be bigger than a SGA full termer but have trouble breathing on his own.
Another classification system uses standard weights. Average BW (birth weight) is 3400g. LBW (low birth weight) means less than 2500g. Very low birth weight (VLBW) means babies with a BW less than 1500g. and extremely low birth weight (ELBW) means babies with a BW less than 1000g. Babies under 500g. do occur and survive (with frequent impairments).
LBW, VLBW and ELBW are not a diagnosis. They are just a convenient label to identify a group of babies with similar types of risks (premature and/or SGA).
Feedings of their own mother's milk is essential for premature babies. It should not be considered optional. Mother's milk is different if the baby is born prematurely than full term in a positive way. It has more of the components that premature babies need. It is also a life saving intervention that only the mother can provide. Already fragile, premature babies are more susceptable to infections when fed on formula.
Even if a mother did not plan to breastfeed she should pump her milk for her infant while in the NICU.
Paula Meier, a well known researcher in the field of breastfeeding premature infants speaks and writes about near term infants as well. She presents evidence that babies of 38 weeks gestation and below have developemental behaviors that mark them as premature. They look full term but they don't sustain their temperature nor feed as well as babies of 39 weeks or more. They are more often readmitted to hospitals because of this. Take home message, those near term 34 to 37 week gestation babies sent home after 2 days in hospital merit closer supervision than the average full term infant.