BIRTH WEIGHT AND GESTATIONAL AGE

At birth, the average weight of the full-term newborn is 3000 to 4000 g. Before 1961, infants weighing less than 2500 g were classified as premature. In 1961, owing to the

recognition that factors other than gestational age affect birth weight, infants weighing less than 2500 g were clas­sified as low birth weight (LBW). Lubchenco and Battaglia established standards for birth weight, gestational age, and intrauterine growth in the United States in the 1960s⁹,¹⁰ (Fig. 2-4). With these standards, gestational age can be as­sessed, and normal and abnormal growth can be identified. The Colorado Growth Curve places newborns into per-centiles.⁹ The 10th through 90th percentiles of intrauterine growth encompass 80% of births.¹¹ Growth is considered abnormal when a newborn falls above or below the 90th and 10th percentiles, respectively.

An infant is considered term when born between the beginning of the 38th week and completion of the 41st week. An infant is considered premature when born before the end of the 37th week and post-mature when born after the end of the 41st week. The lowest mortality rates occur among newborns with weights between 3000 and 4000 g with gestational ages of 38 to 42 weeks.¹²-¹⁴