Definitions
- Small for gestational age (SGA) – an infant with a birth weight <10th centile for its gestational age.
- Severe SGA – a birth weight < 3rd centile.
- Fetal SGA – an estimated fetal weight (EFW), or abdominal circumference (AC) <10th centile.
- Severe fetal SGA – an EFW or AC <3rd centile.
- Fetal growth restriction (FGR) – when a pathological process has restricted genetic growth potential. This can present with features of fetal compromise including reduced liquor volume (LV) or abnormal doppler studies.
- The likelihood of FGR is higher in a severe SGA fetus.
- Low birth weight refers – an infant with a birth weight <2500g.
Aetiology and Pathophysiology
Normal (Constitutionally) Small
50 to 70% of SGA fetuses/infants are constitutionally small, identified by small size at all stages but growth following the centiles. No pathology is present. Contributing factors include ethnicity, sex, and parental height.
Placenta Mediated Growth Restriction
Growth is usually normal initially but slows in utero. This is a common cause of FGR. Maternal factors that can result in placental insufficiency include low pre-pregnancy weight, substance abuse, autoimmune disease, renal disease, diabetes and chronic hypertension.
Non-Placenta Mediated Growth Restriction
Growth is affected by fetal factors such as a chromosomal or structural anomaly, an error in metabolism or fetal infection.
Risk Factors
At booking, and again at 20 weeks gestation, all women should be assessed for risk factors for SGA.
| Minor risk factors | Major risk factors |
| Maternal age ≥35 | Maternal age >40 |
| Smoker 1-10/day | Smoker ≥11/day |
| Nulliparity | Previous SGA baby |
| BMI<20 or 25-34.9 | Maternal/paternal SGA |
| IVF singleton | Previous stillbirth |
| Previous pre-eclampsia | Cocaine use |
| Pregnancy interval <6 or ≥60 months | Daily vigorous exercise |
| Low fruit intake pre-pregnancy | Maternal disease* |
| Heavy bleeding | |
| Low PAPP-A^ |
* Chronic hypertension, renal impairment, diabetes with vascular disease and antiphospholipid syndrome
^PAPP-A = pregnancy associated plasma protein, a hormone produced by the placenta
Diagnosis and Clinical Features
Ultrasound is used for the diagnosis and surveillance of an SGA fetus. Ultrasound biometrics, including EFW and AC, are plotted on customised centile charts. These charts take into account maternal characteristics (height, weight, ethnicity and parity), gestational age and sex.
The ratio of head circumference (HC) and AC may be significant; a symmetrically small fetus is more likely to be constitutionally small whilst an asymmetrically small fetus is more likely to be caused by placental insufficiency. The ‘brain-sparing’ effect can be identified by abnormal doppler studies.
Placental insufficiency can result in impaired fetal kidney function which will result in reduced amniotic fluid volume.
Investigations
Other investigations that may be appropriate include:
- Detailed fetal anatomical survey
- Uterine artery Doppler (UAD)
- Karyotyping
- Screening for infections including congenital cytomegalovirus, toxoplasmosis, syphilis and malaria
Management
Prevention
Modifiable risk factors should be managed to help prevent SGA, including promoting smoking cessation and optimising maternal disease.
Women at high risk for pre-eclampsia should be started on 75mg of aspirin 16 weeks gestation until delivery.
Surveillance
UAD should be the primary surveillance tool in the SGA fetus. If it is normal repeat every 14 days. If it is abnormal repeat more frequently or consider delivery.
Other tests useful in surveillance include symphysis fundal height (SFH), middle cerebral artery (MCA) Doppler, ductus venosus (DV) Doppler, cardiotocography (CTG) and amniotic fluid volume.
Delivery
If delivery is being considered between 24 and 35+6 weeks gestation a single course of antenatal steroids should be given.
The table below demonstrates the indications for delivery by gestation and the recommended mode of delivery.
| Gestation | Indication for delivery | Mode of delivery |
| <37 weeks | Absent/reverse end-diastolic flow on Doppler | C-section |
| By 37 weeks | Abnormal UAD or MCA Doppler | Can offer induction |
| At 37 weeks | Normal UAD | Can offer induction |
Induction for an SGA fetus is associated with a higher rate of C-section. Continuous fetal heart rate monitoring is required from the onset of contractions.
Complications
The use of these customised centile charts has been shown to reduce neonatal morbidity and mortality. Increased morbidity and mortality are most closely associated with FGR. Antenatally, there is an increased risk of stillbirth. Potential neonatal and long-term complications are demonstrated in the table below.
| Neonatal complications | Long-term complications |
| Birth asphyxia | Cerebral palsy |
| Meconium aspiration | Type 2 diabetes |
| Hypothermia | Obesity |
| Hypo-/hyperglycaemia | Hypertension |
| Polycythaemia | Precocious puberty |
| Retinopathy of prematurity | Behavioural problems |
| Persistent pulmonary hypertension | Depression |
| Pulmonary haemorrhage | Alzheimer’s disease |
| Necrotising enterocolitis | Cancer* |
*Breast, ovarian, colon, lung and blood
Summary
- SGA = birth weight/EFW/AC <10th centile
- Severe SGA = birth weight/EFW/AC <3rd centile
- SGA is not always pathological
- If ³3 minor risk factors present refer for UAD
- If major risk factor present refer for serial ultrasound and UAD
- UAD used for surveillance
- If UAD is normal induction can be offered at 37 weeks
- If pre-term delivery planned give course of antenatal steroids
- Complications include stillbirth, birth asphyxia, hypothermia, obesity and cancer