Small for Gestational Age

Small for gestational age is defined as a fetus that measures below the 10th centile for their gestational age. Two measurements on ultrasound are used to assess the fetal size:

  • Estimated fetal weight (EFW)
  • Fetal abdominal circumference (AC)

 

Customised growth charts are used to assess the size of the fetus, based on the mother’s:

  • Ethnic group
  • Weight
  • Height
  • Parity

 

Severe SGA is when the fetus is below the 3rd centile for their gestational age. Low birth weight is defined as a birth weight of less than 2500g.

 

Causes of SGA

The causes of SGA can be divided into two categories:

  • Constitutionally small, matching the mother and others in the family, and growing appropriately on the growth chart
  • Fetal growth restriction (FGR), also known as intrauterine growth restriction (IUGR)

 

Fetal growth restriction (FGR), also known as intrauterine growth restriction (IUGR), is when there is a small fetus (or a fetus that is not growing as expected) due to a pathology reducing the amount of nutrients and oxygen being delivered to the fetus through the placenta.

TOM TIP: It is important to note the difference between small for gestational age (SGA) and fetal growth restriction (FGR). Small for gestational age simply means that the baby is small for the dates, without stating why. The fetus may be constitutionally small, growing appropriately, and not at increased risk of complications. Alternatively, the fetus may be small for gestational age due to pathology (i.e. FGR), with a higher risk of morbidity and mortality.

 

The causes of fetal growth restriction can be divided into two categories:

  • Placenta mediated growth restriction
  • Non-placenta mediated growth restriction, where the baby is small due to a genetic or structural abnormality

 

Placenta mediated growth restriction refers to conditions that affect the transfer of nutrients across the placenta:

  • Idiopathic
  • Pre-eclampsia
  • Maternal smoking
  • Maternal alcohol
  • Anaemia
  • Malnutrition
  • Infection
  • Maternal health conditions

 

Non-placenta medicated growth restriction refers to pathology of the fetus, such as:

  • Genetic abnormalities
  • Structural abnormalities
  • Fetal infection
  • Errors of metabolism

 

Other Signs of Fetal Growth Restriction

There may be other signs that would indicate FGR other than the fetus being SGA, such as:

  • Reduced amniotic fluid volume
  • Abnormal Doppler studies
  • Reduced fetal movements
  • Abnormal CTGs

 

Complications

Short term complications of fetal growth restriction include:

  • Fetal death or stillbirth
  • Birth asphyxia
  • Neonatal hypothermia
  • Neonatal hypoglycaemia

 

Growth restricted babies have a long term increased risk of:

  • Cardiovascular disease, particularly hypertension
  • Type 2 diabetes
  • Obesity
  • Mood and behavioural problems

 

Risk Factors

There are a long list of risk factors for SGA:

  • Previous SGA baby
  • Obesity
  • Smoking
  • Diabetes
  • Existing hypertension
  • Pre-eclampsia
  • Older mother (over 35 years)
  • Multiple pregnancy
  • Low pregnancy‑associated plasma protein‑A (PAPPA)
  • Antepartum haemorrhage
  • Antiphospholipid syndrome

 

Monitoring

The RCOG green-top guidelines on SGA (2013) lists major and minor risk factors. At the booking clinic, women are assessed for risk factors for SGA.

Low-risk women have monitoring of the symphysis fundal height (SFH) at every antenatal appointment from 24 weeks onwards to identify potential SGA. The SFH is plotted on a customised growth chart to assess the appropriate size for the individual woman. If the symphysis fundal height is less than the 10th centile, women are booked for serial growth scans with umbilical artery doppler.

Women are booked for serial growth scans with umbilical artery doppler if they have:

  • Three or more minor risk factors
  • One or more major risk factors
  • Issues with measuring the symphysis fundal height (e.g. large fibroids or BMI > 35)

 

Women at risk or with SGA are monitored closely with serial ultrasound scans measuring:

  • Estimated fetal weight (EFW) and abdominal circumference (AC) to determine the growth velocity
  • Umbilical arterial pulsatility index (UA-PI) to measure flow through the umbilical artery
  • Amniotic fluid volume

 

The local guidelines for the initiation and frequency of ultrasound scans may vary. An example regime is a growth scan every four weeks from 28 weeks gestation. Ultrasound frequency is increased where there is reduced growth velocity or problems with umbilical flow.

 

Management

The critical management steps are:

  • Identifying those at risk of SGA
  • Aspirin is given to those at risk of pre-eclampsia
  • Treating modifiable risk factors (e.g. stop smoking)
  • Serial growth scans to monitor growth
  • Early delivery where growth is static, or there are other concerns

 

When a fetus is identified as SGA, investigations to identify the underlying cause include:

  • Blood pressure and urine dipstick for pre-eclampsia
  • Uterine artery doppler scanning
  • Detailed fetal anatomy scan by fetal medicine
  • Karyotyping for chromosomal abnormalities
  • Testing for infections (e.g. toxoplasmosis, cytomegalovirus, syphilis and malaria)

 

Early delivery is considered when growth is static on the growth charts, or other problems are identified (e.g. abnormal Doppler results). This reduces the risk of stillbirthCorticosteroids are given when delivery is planned early, particularly when delivered by caesarean section. Paediatricians should be involved at birth to help with neonatal resuscitation and management if required.

 

Last updated September 2020
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