What is considered the normal weight for a newborn? The normal weight of a baby who reaches full term between 37 and 40 weeks is 2.7–4.1kg (6 – 9 lbs), with an average weight of 3.5kg (7.7 lbs). A baby who weighs less than 2.5kg (5.5 lbs) is considered to have a low birth weight. About half of all twins and 90% of all triplets are born prematurely, before 37 weeks, and have a low birth weight of under 2.5kg.