Growth of the fetus in the uterus depends on a multitude of intrinsic and extrinsic factors. Optimal fetal growth depends on efficient placental function, adequate provision of energy and growth substrates, appropriate hormonal environment, and adequate room in the uterus. Birth weight variability in a population is primarily determined by maternal heredity, intrinsic fetal growth potential, and environmental factors. Abnormal growth, which can occur at
any time during fetal development, can have immediate and long-term consequences for the infant.
Small for Gestational Age. Small for gestational age (SGA) is a term that denotes fetal undergrowth. SGA is defined as birth weight less than 2 standard deviations below the mean for gestational age, or below the 10th percentile. It often is used interchangeably with intrauterine growth retardation (IUGR). Worldwide, between 30% and 40% of infants born at weights less than 2500 g are SGA. Mortality rates of severely affected SGA infants are five to six times those of normally grown infants of comparable gestational age.
Fetal growth retardation can occur at any time during fetal development. Depending on the time of insult, the infant can have symmetric or proportional growth retardation or asymmetric or disproportional growth retardation. Impaired growth that occurs early in pregnancy during the hyperplastic phase of growth results in symmetric growth retardation. Because mitosis is affected, organs and tissues are smaller as a result of overall decreased cell number. Head circumference, length, and weight usually are represented within similar percentile grids, although the head may be smaller, as in microcephaly.15 This is irreversible postnatally. Causes of proportional IUGR include chromosomal abnormalities, congenital infections, and exposure to environmental toxins.
Impaired growth that occurs later in pregnancy during the hypertrophic phase of growth results in asymmetric growth retardation.12-14 Infants with IUGR due to intrauterine malnutrition often have weight reduction out of proportion to length or head circumference but are spared impairment of head and brain growth.7 Tissues and organs are small because of decreased cell size, not decreased cell numbers. Postnatally, the impairment may be partially corrected with good nutrition.
Maternal, placental, and environmental factors affect fetal growth. Because of the effects on the placenta (it also is undergrown), the risk for perinatal complications is higher. These include birth asphyxia, hyperglycemia, poly-cythemia, meconium (i.e., dark green, mucilaginous newborn stool) aspiration, and hypothermia. The long-term effects of growth retardation depend on the timing and severity of the insult. Many of these infants have developmental disabilities on follow-up examination, especially if the growth retardation is symmetric. They may remain small, especially if the insult occurred early. If the insult occurred later because of placental insufficiency or uterine restraint, with good nutrition catch-up growth can occur, and the infant may attain appropriate growth.
Large for Gestational Age. Large for gestational age (LGA) is a term that denotes fetal overgrowth. The definition of LGA is birth weight greater than 2 standard deviations above the mean for gestation, or above the 90th percentile. The excessive growth may result from a genetic predisposition or may be stimulated by abnormal conditions in utero. Infants of diabetic mothers may be LGA, especially if the diabetes was poorly controlled during pregnancy. Maternal hyperglycemia exposes the fetus to increased levels of glucose, which stimulates fetal secretion of insulin. Insulin increases fat deposition, and the result is a macrosomic (large body
size) infant. Infants with macrosomia have enlarged viscera and are large and plump because of an increase in body fat. Complications when an infant is LGA include birth asphyxia and trauma due to mechanical difficulties during the birth process, hypoglycemia, and polycythemia.12
