Asymmetrical intrauterine growth restriction

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

Please, refer to the article on symmetrical intrauterine growth restriction for a particular discussion of this other type of IUGR. 

Epidemiology

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

Pathology

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

Causes

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

Associations

The incidence of concurrent karyotypic abnormalities is 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 percentile
  • 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 additional sonographic feature.