Can abdominal circumference (AC) alone be relied upon to predict a large baby or shoulder dystocia?

Abdominal Circumference Alone Is Not Reliable for Predicting Large Babies or Shoulder Dystocia

Abdominal circumference (AC) alone should not be relied upon as the sole predictor of macrosomia or shoulder dystocia, as it has limited predictive value and may lead to unnecessary interventions. 1

Limitations of Using Abdominal Circumference Alone

Ultrasound estimation of fetal weight, including measurement of abdominal circumference, has significant limitations:

• According to ACOG guidelines, the diagnosis of fetal macrosomia is imprecise, and the accuracy of estimated fetal weight using ultrasound biometry is no better than clinical palpation (Leopold's maneuvers) 1
• While AC is included in fetal biometry measurements, it should be used in combination with other parameters (biparietal diameter, head circumference, and femur length) to estimate fetal weight 1
• The Society for Maternal-Fetal Medicine specifically recommends that fetal growth restriction be defined using either estimated fetal weight OR abdominal circumference below the 10th percentile, but does not support using AC alone for macrosomia prediction 1

Better Predictors of Macrosomia and Shoulder Dystocia

Multiple factors should be considered when assessing risk for macrosomia and shoulder dystocia:

1. Risk factors for macrosomia:

   • Maternal diabetes (pre-gestational or gestational)
   • Maternal obesity
   • Excessive weight gain during pregnancy
   • History of previous macrosomic infant
   • Post-term pregnancy (>40 weeks)
   • Male fetus 1

2. Better predictive measurements:

   • Complete estimated fetal weight using multiple parameters
   • Fetal asymmetry: A difference of ≥2.6 cm between abdominal diameter and biparietal diameter is associated with increased risk of shoulder dystocia (OR 3.67) 2

Clinical Implications and Management

When macrosomia is suspected:

• ACOG guidelines state that labor and vaginal delivery are not contraindicated for women with estimated fetal weights up to 5,000 g in the absence of maternal diabetes 1
• For diabetic women, prophylactic cesarean delivery may be considered when estimated fetal weight exceeds 4,500 g 1
• For non-diabetic women, prophylactic cesarean delivery may be considered when estimated fetal weight exceeds 5,000 g 1
• Induction of labor for suspected macrosomia is not recommended, as it doubles the risk of cesarean delivery without reducing shoulder dystocia risk 1

Common Pitfalls to Avoid

1. Overreliance on single measurements: Using AC alone can lead to false positives and unnecessary interventions
2. Inaccurate weight estimation: Ultrasound estimation of fetal weight has an error margin of ±10-15%
3. Unnecessary induction: Inducing labor for suspected macrosomia increases cesarean delivery rates without improving outcomes
4. Failure to consider maternal diabetes: The risk of shoulder dystocia is significantly higher in diabetic pregnancies with macrosomia (19.9-50%) compared to non-diabetic pregnancies (9.2-24%) 1

In conclusion, while abdominal circumference is a component of fetal weight estimation, it should not be used in isolation to predict macrosomia or shoulder dystocia. A comprehensive assessment including multiple biometric parameters, maternal risk factors, and consideration of fetal asymmetry provides a more accurate risk assessment for clinical decision-making.