Small for Gestational Age Health Article

Small for Gestational Age

Small for gestational age (SGA) describes infants who weigh less than they should given their gestational age (the weeks they have spent in the uterus).

SGA infants can be premature, full-term, or post- term; such infants are simply not as large as would be predicted by their maturity. Experts differ on whether the cutoff for being small for gestational age (SGA) should be the third percentile or the tenth percentile for weight. Being SGA is not a disease in itself, but rather a risk factor for developing problems during the perinatal period as a result of having experienced smaller-than-expected growth in the womb.

Causes

Although underlying fetal problems cause some infants to be SGA, in the vast majority of SGA infants, the condition is caused by an inadequate supply of oxygen or nutrients before birth. Uterine environmental problems that can lead to SGA include:

• illness in the mother, such as severe high blood pressure, severe heart disease or vascular disease, or gastrointestinal ailments severe enough to result in malnutrition in the mother
• toxins such as smoking, alcohol, and certain abuse of drugs
• poor transport of oxygen to the fetus, such as infants born at high altitude, or mothers who have unusual types of hemoglobin (i.e., sickle-cell hemoglobin).

These uterine environment problems are by far the most common causes of underweight infants, but a number of fetal problems may also cause inadequate growth. These problems include chromosomal abnormalities (e.g., Turner syndrome, trisomy 21); certain malformation syndromes (e.g., dwarfism, diGeorge syndrome); and multiple gestations, including twinning. (Even healthy twins, however, are smaller on average than single births of the same gestational age; additionally, sometimes there will be a condition known as twin-twin transfusion, in which one twin receives most of the blood supply from the placenta and the second twin is shortchanged and thus stunted.)

Complications

With improved methods of prenatal fetal monitoring, many cases of intrauterine growth retardation can be ascertained prior to delivery. This is important because a number of perinatal problems must be anticipated and dealt with promptly in an SGA newborn.

SGA babies have poor fat supplies and poor stores of glycogen (an energy-storing molecule). They are thus prone to hypoglycemia (low blood sugar) in the hours after birth and often have trouble maintaining a normal body temperature. SGA infants also have high rates of intra-partum distress and asphyxia at the time of birth because they do not tolerate the stresses of labor and delivery as well as infants who are appropriate for gestational age (AGA). Subtle neurological problems may be present at the time of birth, including tremor and abnormal reflex patterns. SGA infants may also have increased blood viscosity and calcium abnormalities in the hours following birth.

Prognosis

Long-term prognosis depends on the severity of the growth retardation they experience, the timing of the intrauterine insult, and whether brain growth (as measured by head growth) is affected or relatively spared.

—Marta M. Vielhaber, M.D.