FETAL PROPORTIONS

Fetal proportion is based on the comparison of fetal head size to abdominal size - fetal head to abdomen ratio (1).

• Normal fetus
• Head grows faster than the abdomen until 36 weeks gestation.
• >36 weeks, abdominal growth slightly exceeds head growth.
• HC/AC ratio falls with gestational age due to the increase rate of liver growth.
• Dysmature IUGR
• Placental insufficiency results in a diminished liver size due to decreased glycogen deposition while head size and growth is maintained ("brain-sparing effect").
• Providing there are no severe fetal anomalies that affect the AC and HC an increase HC/AC ratio occurs.
• Predictive value of the HC/AC ratio:
• Crane and Kopta (2). Select group of fetuses at risk for dysmature IUGR, there was a perfect predictive accuracy. Of the 37 patients (79%) with a normal ratio, none had IUGR, whereas of the remaining 10 patients (21%) with an elevated ratio, all had IUGR.
• Divon and associates (3). In an IUGR population of mixed etiologies, a lower predictive accuracy was found. Sensitivity 36%, Specificity 90%, positive predictive value of 67%, negative predictive value of 72%.
• Manning and co-workers (4). The predictive value of the HC/AC ratio is strongly influenced by the type and severity of the IUGR, as well as the gestational age of the fetus being tested.
  Dysmature IUGR in a fetus >28 weeks, sensitivity >93%, but sensitivity decreases inversely with gestational age and disease severity. Dysmature IUGR in a fetus <26 weeks, may exhibit paradoxical findings of head to abdomen symmetry.
  The explanation for this phenomenon is unknown but thought to be related to an immature "cardiac output redistribution adaptive reflex". The HC/AC ratio cannot be used to assess the anomalous fetus.

Symmetrical IUGR	Asymmetrical IUGR
Equal growth Normal and small Abnormal and small	Redistribution of growth Dysmature

REFERENCES

1. Campbell S. Ultrasound measurement of the fetal head to abdominal surface ratio in assessment of growth retardation. Br J Obstet Gynaecol 1977;84:165.
2. Crane JP, Kopta MM. Prediction of intrauterine growth retardation via ultrasonically measured head/abdomen circumference ratios. Obstet Gynecol 1979;54:597.
3. Divon MY, Guidetti DA, Braverman JJ et.al. Intauterine growth retardation: A prospective study of the diagnostic value of real time sonography combined with umbilical artery flow velocimetry. Obstet Gynecol 1988;72:611.
4. Manning FA. Fetal Medicine: Principles and Practice. Appleton and Lange, Connecticut 1995:349-351.