Fetal Clavicular Measurement to Predict Fetal Macrosomia: A Prospective Cohort Study

Two terms are applied for fetal overgrowth, Large for gestational age (LGA) meaning fetal birth weight (BW) more than 90th percentile for specific gestational age while macrosomia is an absolute value regardless of gestational age which historically defined as 4000-4500 gm. Those two groups have increased risks for neonatal and maternal complications compared to general population and increase sharply when BW >4500gm, the risks of macrosomia are continuum without threshold defining safe and risky outcome, some authors classify macrosomia into 3 grades, grade 1 (4000gm-4499gm), grade 2(4500-4999), grade 3 (≥5000gm).

Despite its implications, the accurate diagnosis is after birth and its prenatal prediction is poor although published formulas for estimating fetal weight shows correlation with BW, however the variability of the estimate is up to 20% with most of formulas, meta-analysis of 29 studies showed sensitivity of 56% and specificity of 92% in predicting BW ≥ 4000gm accuracy of ultrasound decreases with increasing BW, BW>4500 accurate prediction is only 33-44 % of cases. Given the poor predictability of macrosomia, variety of other techniques and formulas are investigated, neither repeated US examination nor growth curves improves predictability, Youssef's formula measuring biacromial diameter (distance by between both acromial processes which joins clavicles at acromioclavicular joints) and macrosomic specific formula seems to be predictive. In study evaluating clavicle length for shoulder dystocia, it found that measuring clavicle was significant for macrosomia however the limitation is small sample size and its comparison with other fetal biometrics may be needed.