The normal weight of a newborn oscillates according to gestational age and there are reference values and percentiles that determine the appropriate weight. Exceeding or not reaching these reference values can sometimes trigger complications for the newborn that require closer pediatric follow-up or for the mother.
Birth weight is defined as the weight of an infant immediately after birth. This value is directly related to the age at which the baby was born and can be estimated during pregnancy by measuring the fundal height. Most babies born between 37 and 40 weeks gestation (term birth) weigh between 2.5 and 4 kg. Below or above this range is considered underweight or overweight, respectively.
A neonate within the normal range of weight for gestational age is considered appropriate for gestational age (AEG), whereas a neonate born above or below the defined gestational age limit will be exposed to fetal development that predisposes it to complications for both its health and that of its mother.
There are several factors that can affect the size of a baby at birth, such as the length of pregnancy, the size and height of the parents, multiple pregnancies, the order of birth, and the size of the fetus.There are several factors that can affect the size of a baby at birth such as the length of the pregnancy, the size and height of the parents, multiple pregnancies, birth order (firstborns are sometimes smaller), gender (girls tend to be smaller), or the mother's health during pregnancy.
Maternal factors that can lead to lower birth weight include hypertension, heart problems, smoking, alcohol or drug use. If the mother has diabetes or obesity, the baby may weigh more at birth (macrosomia). In addition, excessive maternal weight gain during pregnancy may increase the chances that a baby will be born at a higher than average weight.
Identification of small-for-gestational-age (SGA) children is important because they are at increased risk for perinatal morbidity and cardiovascular disease in adulthood. Small-for-gestational-age infant is defined as a newborn who presents length and/or birth weight < 2 SD (standard deviations) or P3 (percentile 3) for gestational age. On the other hand, fetal macrosomia is associated with a higher incidence of cesarean section and mortality is higher. Several studies link fetal macrosomia in children of diabetic, obese mothers and even in normal population, with an increased risk of developing type 2 diabetes mellitus (DM2), obesity and metabolic syndrome in childhood or adulthood.
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Kids Health. Nemours Foundation [May 2022]
American Academy of Pediatrics. Tracking Your Baby's Weight and Measurements [May 2022]
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Warrington NM et al. Maternal and fetal genetic effects on birth weight and their relevance to cardio-metabolic risk factors. Nature Genetics, 01 May 2019, 51(5):804-814.