Ultrasound Assessment of Fetal Macrosomia
  1. Estimated Fetal Weight (EFW):
    • There is a large standard deviation in mean differences of actual versus estimated fetal weight (1).
    • Sonographic estimated fetal weight is a poor predictor of actual fetal weight. Predictive value is only 64% (2).
    • Most formulae for estimated fetal weight overestimate birth weight.

 
  1. Estimated Fetal Weight + Abdominal Circumference (AC).
    • If EFW + AC exceeds the 90th percentile, macrosomia is correctly diagnosed in 88.8% of fetuses (3).
    • It appears that AC growth is accelerated from 32 weeks in a group of large for gestational age (LGA) fetuses (4).

 
  1. Abdominal Circumference (AC) (4):
    • Grows at a rate of ± 1.2 cm / week is an optimal cut-off point for detecting LGA infants (4). Sensitivity = 83.8%, specificity = 85.4%, positive predictive value = 78.8% and negative predictive value = 89% (4).
    • AC of >35 cm may identify >90% of fetuses with macrosomia that are at risk for shoulder dystocia (5).
    • AC minus BPD of 2.5cm or more predicted all cases of shoulder dystocia in one series (6) but was not predictive in another series (5).
    • AC of >2 standard deviations is also a good predictor of LGA infant

 
  1. FL/AC Ratio:

Ratio correctly identified 89% of LGA fetuses compared to 63% in non-diabetic fetuses.
  1. HC/AC Ratio:

            Gestational age dependent (accurate gestational age is essential).
  1. Fetal Soft Tissues:

 

Using all the above data it is possible to over-react to false information and therefore increase the rate of cesarean section.
One worker suggests that in a diabetic mother, a sonographic EFW of over 4200 grams with morphometric changes of increased AC in comparison to HC and FL should be used to persuade the clinician to do an elective cesarean section (8).

 

 

 

 

REFERENCES

 

  1. Benson CB, Doubilet PM, Saltzman DH. Sonographic determination of fetal weights in diabetic pregnancies. Am J Obstet Gynecol 1987;156:441-444.
  2. Pollack RN, Hauser-Pollak G, Divon MY. Macrosomia in postdates pregnancies: the accuracy of routine ultrasonographic screening. Am J Obstet Gynecol 1992;167:7-11.
  3. Tamura RK, Sabbaha RE, Depp R et.al. Diabetic macrosomia: accuracy of third trimester ultrasound. Obstet Gynecol 1986;67:828.
  4. Landon MB, Marshall C, Mintz et.al. Sonographic evaluation of fetal abdominal growth: predictor of the large- for-gestational-age infant in pregnancies complicated by diabetes mellitus. Am J Obstet Gynecol 1990;160:115-121.
  5. Jazayeri A, Heffron JA, Phillips R et.al. Macrosomia prediction using ultrasound fetal circumference of 35 centimeters or more. Obstet Gynecol 1999;93:523-526.
  6. Cohen B, Penning S, Major C et.al. Sonographic prediction of shoulder dystocia in infants of diabetic mothers. Obstet Gynecol 1996;88:10-13.
  7. Santolaya-Forgas J, Meyer WJ, Gauthier DW et.al. Intrapartum fetal subcutaneous tissue / femur length ratio: an ultrasonographic clue to fetal macrosomia. Am J Obstet Gynecol 1993;171:1072-1075.
  8. Coetzee EJ. Macrosomia. In: Textbook of Fetal Ultrasound. Jaffee R, The-Hung B (eds). Parthenon Publishers, London 1999:81-86.