Small for gestational age

Small for gestational age (SGA) babies are those whose birth weight lies below the 10th percentile for that gestational age. They have usually been the subject of intrauterine growth restriction (IUGR), formerly known as intrauterine growth retardation.^[1] Low birth weight (LBW), is sometimes used synonymously with SGA, or is otherwise defined as a fetus that weighs less than 2500 g (5 lb 8 oz) regardless of gestational age. Other definitions include Very Low Birth Weight (VLBW) which is less than 1500 g, and Extremely Low Birth Weight (ELBW) which is less than 1000 g.^[2]

There is a 8.1% incidence of low birth weight in developed countries, and 6–30% in developing countries. Much of this can be attributed to the health of the mother during pregnancy. One third of babies born with a low birth weight are also small for gestational age.

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Diagnosis

The condition is generally diagnosed by measuring the mother's uterus, with the fundal height being less than it should be for that stage of the pregnancy. If it is suspected, the mother will usually be sent for an ultrasound to confirm.

Predetermining factors

The risk factor/etiology can be broadly divided into 3 categories-

• Fetal
• Maternal
• Placental

The primary risk factor is that development of the placenta is insufficient to meet the demands of the fetus, resulting in malnutrition of the developing fetus. There are numerous contributing factors, of both environmental and genetic origin:

• Environmental factors such as poor nutrition, tobacco smoking, drug addiction or alcoholism
• Severe anaemia (although hydrops may also occur)
• Thrombophilia (tendency for thrombosis)
• Prolonged pregnancy
• Pre-eclampsia
• Chromosomal abnormalities
• Damaged or reduced placental tissue due to:
  • Chronic renal failure
  • Sickle cell anemia
  • Phenylketonuria
• Infections such as rubella, cytomegalovirus, toxoplasmosis or syphilis
• Twins and multiple births.

Categories of growth restriction

There are two distinct categories of growth restriction, indicating the stage at which the development was slowed. Small for gestational age babies can be classified as having symmetrical or asymmetrical [asymmetrical] growth restriction.^[3][4]

Symmetrical

Symmetrical growth restriction, less commonly known as global growth restriction, indicates that the fetus has developed slowly throughout the duration of the pregnancy and was thus affected from a very early stage. The head circumference of such a newborn is in proportion to the rest of the body. Common causes include:

• Early intrauterine infections, such as cytomegalovirus, rubella or toxoplasmosis
• Chromosomal abnormalities
• Chronic high blood pressure
• Severe malnutrition
• Anemia
• Maternal substance abuse (prenatal alcohol use can result in Fetal alcohol syndrome)

Asymmetrical

Asymmetrical growth restriction occurs when the embryo/fetus has grown normally for the first two trimesters but encounters difficulties in the third, usually pre-eclampsia. Such babies have a disparity in their length and head circumference when compared to the birth weight. A lack of subcutaneous fat leads to a thin and small body out of proportion with the head. Other symptoms include dry, peeling skin and an overly-thin umbilical cord, and the baby is at increased risk of hypoxia and hypoglycaemia.

Treatment

Possible treatments include the early induction of labour, though this is only done if the condition has been diagnosed and seen as a risk to the health of the fetus.

References