WHAT IS IT?

The term fetal macrosomia implies growth beyond an absolute birth weight of 4000 grams or 4500 grams, regardless of gestational age. The risk of morbidity for both infants and mothers increases when the birthweight is between 4000 and 4500 grams. Risks for maternal and newborn morbidity rise considerably with birthweights >4500g. A correct diagnosis can only be made after weighing an infant at birth, as ultrasound prediction is not precise.

CLINICAL ACTIONS:

• Consider a prophylactic cesarean for suspected fetal macrosomia if the EFW (estimated fetal weight) is > 5000 grams in women without diabetes
• Consider a prophylactic cesarean for suspected fetal macrosomia if the EFW is > 4500 grams in women with diabetes
• Induction before 39w0d is not suggested for suspected fetal macrosomia as induction has not been shown to improve maternal or fetal outcomes
• Suspected fetal macrosomia is not a contraindication to a trial of labor after cesarean

SYNOPSIS:

In the United States, 7.8% of all live-born infants weigh > 4000 grams and 1% weigh > 4500 grams. The most serious complication of fetal macrosomia is shoulder dystocia which complicates 0.2-3.0% of all vaginal deliveries. When the birthweight is > 4500 grams, the shoulder dystocia rate increases to 9-14%. The shoulder dystocia rate increases to 20-50% in the presence of maternal diabetes when the birthweight is > 4500 grams.  ‘Large for gestational age (LGA)’ also refers to excessive fetal growth, but rather than absolute weight, LGA is usually defined as ≥90th percentile for a given gestational age.

KEY POINTS:

Risk Factors for Macrosomia

• Preexisting maternal diabetes
• Uncontrolled gestational diabetes
• Excessive gestational weight gain
• Excessive inter pregnancy weight gain
• Prior macrosomic infant
• Post-term pregnancy
• Maternal nonsmoking status

Maternal Risks Associated with Macrosomia

• Increased risk of cesarean delivery
• Postpartum hemorrhage
• Vaginal lacerations

Fetal Risks Associated with Macrosomia

• Shoulder dystocia leading to brachial plexus injury or clavicular fracture
• Decreased 5 minute Apgar score
• Increased rates of admission to the NICU, including longer stays
• Obesity later in life

Accuracy of EFW Measurement

• Fundal height
  • “Poor predictor of macrosomia”: Better at ruling out than identifying macrosomia
  • Sensitivity: 29% to 70%
  • Specificity: >90%
• Abdominal palpation maneuvers
  • Sensitivity: 16% to 68%
  • Specificity: 90% to 99%
  • PPV: 38% to 80%
• In women with diabetes
  • “Clinical estimates of macrosomia are as predictive as those derived with ultrasonography”
• Ultrasound
  • Prediction of birth weight >4,500g
    • Sensitivity: 10% to 45%
    • Specificity: 57% to 99%
    • PPV: 11% to 44%
    • NPV: 92% to 99%
  • Prediction of birth weight >4,000g
    • Sensitivity: 56%
    • Specificity: 92%
  • Hadlock estimation
    • Newborns>4,500 g: Mean absolute percent error of 13% | Increases with greater EFW
    • Nonmacrosomic newborns: Mean absolute percent error of 8%

Note: Upon review of current literature, ACOG states

No single formula based on ultrasound biometry performs significantly better than others for the detection of macrosomia more than 4,500 g

Similar to clinical estimates of fetal weight, ultrasonography can be used most effectively as a tool to rule out macrosomia, which may help avoid maternal and fetal morbidity

Learn More – Primary Sources:

ACOG Practice Bulletin No. 216 : Macrosomia

Related ObG Topics:

Gestational Diabetes Mellitus – Definitions, Risk Factors and Complications

Does Early Accelerated Fetal Growth Increase the Risk for LGA and Macrosomia?

Obesity in pregnancy – classification and clinical implications

Shoulder Dystocia: Diagnosis, Evaluation and Management

Updated ACOG Guidance on Gestational Diabetes