Birth weight is the first weight of the fetus or newborn 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. While statistical tabulations include 500 g groupings for birth weight, weights should not be recorded using those groupings; instead the actual weight should be recorded to the degree of accuracy to which it is measured.
At the lower end several birth weight categories can be defined:
- low birth weight: less than 2500 g (up to and including 2499 g);
- very low birth weight: less than 1500 g (up to and including 1499 g);
- extremely low birth weight: less than 1000 g (up to and including 999 g).
Note, however, that these definitions do not constitute mutually exclusive categories: below the set limits they are all-inclusive and therefore overlap (i.e. 'low' includes 'very low' and 'extremely low', while 'very low' includes 'extremely low').