Small for Gestational Age (SGA) Babies

Synonyms: Light-for-date babies.

Small for gestational age (SGA) babies are those weighing ≤ 2500g if born at term. They fall into 2 groups:

• Babies whose growth at all gestational ages has been low. They are light-for-dates, but otherwise healthy.
• Growth is normal in early part of pregnancy, but slows in utero by at least 2 measurements, normally ultrasound. This is due to intra-uterine growth restriction. They have a wasted appearance with little subcutaneous fat, are at greater risk of dying.

The very low birth weight (VLBW) baby weighs ≤1500g.

Significance of SGA

SGA fetuses are at greater risk of stillbirth, intrapartum hypoxia, neonatal complications, impaired neurodevelopment and possibly type 2 (non-insulin-dependent) diabetes and hypertension in adult life.^1,2,3,4 However, the vast majority of term SGA infants have no appreciable morbidity or mortality.⁵

Incidence

7% of neonates are of low birth weight. 10% weigh ≤ 2500g.⁶

Risk factors

• Maternal age ≤17 years or ≥35 years
• Socio-economic class IV or V⁷
• Weight before pregnancy ≤50 kg or ≥75 kg⁸
• Smoking⁹
• Excessive consumption of alcohol^10,11
• Drug/substance abuse
• Previous low birth weight baby
• Anaemia in pregnancy
• Chronic hypertensive disease¹²
• Maternal polycystic ovarian syndrome¹³
• Ante-partum haemorrhage
• Multiple pregnancy
• Congenital fetal defects
• Intra-uterine infection

There are a variety of methods used to detect SGA fetuses:

• Abdominal palpation
• Measurement of symphyseal fundal height
• Ultrasound - estimated fetal weight
• Ultrasound - biophysical profiling
• Ultrasound - doppler flow velocimetry

However, the following should not be forgotten:

• Most investigations use a one-off measurement (size) to predict SGA, while there is evidence that it is the trend (growth) that is of more value in predicting poor fetal outcome.
• All require an accurate estimation of gestation as a prerequisite for accuracy.
• Most tests diagnose SGA fetuses, rather than growth-restricted fetuses.
• Little allowance is made for important prognostic factors for SGA, such as maternal height, weight, ethnicity, parity and fetal gender.

Antenatal

• If a SGA fetus is found, there should be a careful ultrasound survey for chromosome defects. Karyotyping may be offered.¹⁴
• The fetus should be monitored with umbilical artery doppler. A study comparing fetal heart-rate monitoring, biophysical profile and umbilical artery doppler found that only umbilical artery doppler had value in predicting poor perinatal outcomes in SGA fetuses.¹⁵ Absent or reversed end diastolic flow is associated with increased perinatal mortality and morbidity.^16,17
• Timing delivery is a balance between doppler findings and gestation. If the fetus is less than 36 weeks gestation, the mother should receive corticosteroids.
• The fetus should be delivered in a unit with neonatal expertise and facilities.

Intrapartum

• Continuous electronic fetal monitoring should be offered.¹⁸
• There is currently no evidence to support routine elective caesarean delivery.^19,20

Post delivery

• Care should either be provided for in a nursery temperature >24 °C, or for very small babies in an incubator at a temperature of 26-32 °C and humidity 65-75%.
• Temperature is decided by monitoring infant's core temperature and adjusting accordingly.
• Oxygen is supplied either by a head box or into the incubator:
  • The level is adjusted according to monitoring by transcutaneous oxygen electrode or pulse oximetry.
  • A level is maintained that avoids hypoxia, but also considers the problem of toxicity on the retina (retinopathy of prematurity).²¹
• Infection control is important:
  • Careful attention should be paid to avoiding overcrowding and possibility of medical attendants or relatives introducing infection into the nursery.
  • Hands must be washed before handling a baby.
• Feeding should start 6 hours after birth, using breast milk as soon as possible:
  • Normally baby is fed by nasogastric tube or spoon.
  • They are put to the breast as soon as suckling strongly.
  • If baby is unable to accept, enteral feeding then may need to supplement with parenteral nutrition.
  • Regularly check for hypoglycaemia and treat with IV glucose if necessary.
• Supplements of vitamin A, C, D, niacin and riboflavin are also given.²²
• If Ca²⁺ <2mmol/l, give calcium gluconate IV slowly.
• Cranial ultrasound (to detect haemorrhage)

• Spontaneous preterm delivery²³
• Respiratory distress syndrome
• Cyanotic attacks - caused either by inadequate ventilation or cerebral damage
• Jaundice
• Hypoglycaemia - affects 15% of cases
• Hypocalcaemia - usually occurs in first 24 hours of life
• Intracranial haemorrhage - mainly occurs in babies weighing <1500g
• Retinopathy of prematurity and sequelae²⁴
• Cerebral palsy²⁵

• Mortality increases with decreasing weight.⁶
• Preterm, low birth weight infants have an extremely high risk of mortality in their first year and parents should be counselled regarding this risk.²⁶

VLBW females catch up in growth by 20 years of age whereas VLBW males remain significantly shorter and lighter than controls.²⁷

• Being underweight and preterm leads to poor weight gain and head growth in infancy.²⁸
• Low birth weight is related to early developmental delay and later language problems.²⁹
• However, neonatal complications may have a larger detrimental effect on long term cognitive development of these infants, than their birthweight alone.
• The rate of neurological abnormalities and the rate of deafness has increased with improved survival rates for these babies.

NB: Prospective parents of extremely low birth weight infants should be advised of these substantial risks.³⁰