The Symmetric vs. Asymmetric Classification of FGR is Clinically Obsolete
The distinction between symmetric and asymmetric fetal growth restriction should no longer be used in clinical practice, as it does not predict outcomes or guide management decisions. 1, 2
Why This Classification Has Been Abandoned
The most authoritative current guidelines explicitly state that while FGR was historically classified as symmetric or asymmetric based on the head circumference to abdominal circumference (HC/AC) ratio, this classification is not prognostic of outcome and should no longer be used as a description of FGR 2.
The 2020 Society for Maternal-Fetal Medicine guidelines reinforce this position, noting that:
- HC/AC ratio was not found to be an independent predictor of adverse pregnancy outcomes 1
- Growth and developmental delay from birth to age 4 years were shown to be similar in symmetric and asymmetric growth-restricted preterm newborns 1
- The classification was thought to provide information about timing of insult and etiology, but recent evidence has disproven this assumption
What Actually Matters for Clinical Decision-Making
Instead of the symmetric/asymmetric distinction, focus on factors that actually predict morbidity, mortality, and quality of life:
Timing of Onset (Early vs. Late)
- Early-onset FGR (diagnosed <32 weeks): More severe, greater placental dysfunction, higher mortality risk
- Late-onset FGR (diagnosed ≥32 weeks): Represents 70-80% of cases, typically milder presentation 1
Severity Markers
- Estimated fetal weight <3rd percentile: 3-fold increased stillbirth risk compared to 3rd-5th percentile 1
- Abnormal umbilical artery Doppler: Predicts adverse perinatal outcomes 1
- Abnormal ductus venosus waveforms: Indicates severe cardiovascular compromise
Common Pitfall to Avoid
Do not waste time calculating HC/AC ratios to classify FGR as symmetric or asymmetric. This outdated approach diverts attention from the surveillance and timing decisions that actually impact outcomes. The research showing asymmetric patterns in early-onset severe FGR 3 is descriptive only and does not change management.