Asymmetrical intra-uterine growth restriction

Asymmetrical intra-uterine growth restriction is a type of intra uterine growth restriction (IUGR) where some fetal biometric parameters are disproportionately lower than others as well as falling under the 10^th centile. The parameter classically affected is the abdominal circumference (AC).

Epidemiology

It tends to present more in the 3^rd trimester (later than the symmetrical IUGR pattern) and is the more commoner of the two main types of IUGR.

Pathology

In this type there is classically relative preservation of the fetal brain (fetal head sparing theory) which pathologically characterised by an increased brain to liver ratio (BLR) ². This can also result in decreased fetal subcutaneous fat ^4,6. A paradoxical rare situation is with maternal cocaine use where the head circumference is reduced out of proportion to others ⁵.

Causes

• placental insufficiency : one of the commonest causes asymmetrical IUGR
• pre-eclampsia  ⁶ :

Associations

The incidence of concurrent karyotypic abnormalities are low (or minimal ³) especially if asymmetrical IUGR is detected late in pregnancy. 

Syndromes that can give an asymmetrical IUGR picture include

• Russell-Silver syndrome

Radiographic features

Antenatal ultrasound

Features in the classical situation include

• changes in fetal biometric parameters
  • fetal abdominal circumference (AC) classically reduced out of proportion to other fetal biometric parameters and is below the 10^th centile
  • certain fetal biometric parameters such as biparietal diameter (BPD) and head circumference (HC) may be normal
  • increased HC : AC ratio
• fetal tachycardia (may be present in up to 50% of cases ⁴)

Oligohydramnios may be present as an ancillary sonographic feature